Add rust_doc_test to ffi example

[sebas-zip]

adding rust_doc_test to ffi example to show linking error. This may be related to the issue:
#78

bazel test //rust_calling_c/example_2:matrix_doc_test
INFO: Analyzed target //rust_calling_c/example_2:matrix_doc_test (0 packages loaded, 0 targets configured).
INFO: Found 1 test target...
FAIL: //rust_calling_c/example_2:matrix_doc_test (see /home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/execroot/_main/bazel-out/k8-fastbuild/testlogs/rust_calling_c/example_2/matrix_doc_test/test.log)
INFO: From Testing //rust_calling_c/example_2:matrix_doc_test:
==================== Test output for //rust_calling_c/example_2:matrix_doc_test:

running 1 test
test rust_calling_c/example_2/src/matrix.rs - Matrix (line 30) ... FAILED

failures:

---- rust_calling_c/example_2/src/matrix.rs - Matrix (line 30) stdout ----
error: linking with `/usr/bin/gcc` failed: exit status: 1
  |
  = note: LC_ALL="C" PATH="external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/rust_toolchain/lib/rustlib/x86_64-unknown-linux-gnu/bin" VSLANG="1033" "/usr/bin/gcc" "-m64" "/tmp/rustciT4foi/symbols.o" "/tmp/rustdoctestqoc7oG/rust_out.rust_out.a55a08873e1b84b0-cgu.0.rcgu.o" "/tmp/rustdoctestqoc7oG/rust_out.48ehprphb7s9hlkp.rcgu.o" "-Wl,--as-needed" "-L" "external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/rust_toolchain/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-L" "external/_main~_repo_rules~libc" "-L" "rust_calling_c/example_2" "-L" "external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/rust_toolchain/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-Wl,-Bstatic" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/execroot/_main/bazel-out/k8-fastbuild/bin/rust_calling_c/example_2/libmatrix-1130108680.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/execroot/_main/bazel-out/k8-fastbuild/bin/external/_main~_repo_rules~libc/liblibc-1144681119.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-d5189b81a4fa4d36.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libpanic_unwind-6a4779412a873200.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libobject-c88c426dd6780435.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libmemchr-e74540b31113a555.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libaddr2line-8751b61bd13c15cf.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libgimli-a8b99dba9f449259.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_demangle-f0068d76172a0372.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd_detect-c16dfaf47799564e.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libhashbrown-bfa26dd63e299db5.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_std_workspace_alloc-75a2330a693e738f.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libminiz_oxide-76779dce1f7ab63d.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libadler-01c5cc588623cb35.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libunwind-07bb9745ec737292.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcfg_if-6eacdcc91004cefb.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-ae2488b58226c836.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-52acaddcaaba04c6.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_std_workspace_core-326b78eac9ecd050.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-307ebf19f0f13d30.rlib" "/home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcompiler_builtins-d9076ee5964191bf.rlib" "-Wl,-Bdynamic" "-lc" "-lm" "-lrt" "-lutil" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "external/rules_rust~~rust~rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/rust_toolchain/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-o" "/tmp/rustdoctestqoc7oG/rust_out" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-nodefaultlibs" "-fuse-ld=gold" "-B/usr/bin" "-Wl,-no-as-needed" "-Wl,-z,relro,-z,now" "-pass-exit-codes" "-Wl,--push-state,-as-needed" "-lstdc++" "-Wl,--pop-state" "-Wl,--push-state,-as-needed" "-lm" "-Wl,--pop-state"
  = note: /home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/execroot/_main/bazel-out/k8-fastbuild/bin/rust_calling_c/example_2/libmatrix-1130108680.rlib(matrix-1130108680.matrix.975fc0e578b15861-cgu.0.rcgu.o):matrix.975fc0e578b15861-cgu.0:function matrix::Matrix::new: error: undefined reference to 'matrix_new'
          /home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/execroot/_main/bazel-out/k8-fastbuild/bin/rust_calling_c/example_2/libmatrix-1130108680.rlib(matrix-1130108680.matrix.975fc0e578b15861-cgu.0.rcgu.o):matrix.975fc0e578b15861-cgu.0:function <matrix::Matrix as core::ops::drop::Drop>::drop: error: undefined reference to 'matrix_free'
          collect2: error: ld returned 1 exit status

  = note: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
  = note: use the `-l` flag to specify native libraries to link
  = note: use the `cargo:rustc-link-lib` directive to specify the native libraries to link with Cargo (see https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-link-lib)

error: aborting due to 1 previous error

Couldn't compile the test.

failures:
    rust_calling_c/example_2/src/matrix.rs - Matrix (line 30)

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.12s

================================================================================
Target //rust_calling_c/example_2:matrix_doc_test up-to-date:
  bazel-bin/rust_calling_c/example_2/matrix_doc_test.rustdoc_test.sh
INFO: Elapsed time: 0.311s, Critical Path: 0.21s
INFO: 2 processes: 2 linux-sandbox.
INFO: Build completed, 1 test FAILED, 2 total actions
//rust_calling_c/example_2:matrix_doc_test                               FAILED in 0.2s
  /home/sesovero/.cache/bazel/_bazel_sesovero/1d73c83305bc70be2f9db58ce66d559e/execroot/_main/bazel-out/k8-fastbuild/testlogs/rust_calling_c/example_2/matrix_doc_test/test.log

Executed 1 out of 1 test: 1 fails locally.

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