Docs: improve "Workflows" section for developers

[EZoni]

Suggestion for reviewers:
Compare the preview of the new documentation available from the CI check docs/readthedocs.org:warpx with the existing documentation available online.

Improve the Workflows documentation for developers, so far only the following sections:

• "How to test the code"
• "How to run the clang-tidy linter"

To-do:

• Incorporate minimal checksum documentation in one single test section
• Add step-by-step guide on how to add new tests (enumerated list of steps)

[EZoni]

FYI, this file was actually renamed from run_clang_tidy_locally.rst and modified later on, but the number of changes is large enough to make GitHub display it as a new file.