Document C code API

[npetersen2]

This issue comes from the AMDC-Firmware repo: Severson-Group/AMDC-Firmware#133

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Now that the C code is maturing (i.e. ./drv/foo.c), we should make detailed documents about how to use them from a user perspective.

This should include:

For drivers:

1. Brief explanation of how the internals of the driver work. Should link to the IP core docs if appropriate.
2. Index of all available function calls and what they do.
3. Example code which exercises the driver.

Similar for system code.

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I think we should make use of the Sphinx autodocs feature, but this will take a bit of work to figure out how to robustly use... Since the code is in a different repo. So, might have to make a submodule in this repo, docs.amdc.dev, then, on docs build, the build scripts checkout a given release branch of the AMDC-Firmware and then auto-build the API docs...

This will involve a big push in the source C file documentation with lots of comments added, etc.