Include C construct directives for RST checks

[hugovk]

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Fixes https://github.com/sphinx-contrib/sphinx-lint/issues/108.

Add the "cross-referencing C constructs" from:

https://www.sphinx-doc.org/en/master/usage/domains/c.html#cross-referencing-c-constructs

The 3.13/Ubuntu job fails as expected, because it also checks the friend projects, and finds the two CPython examples mentioned in the issue:

sphinx-lint/tests/fixtures/friends/cpython/Doc/c-api/function.rst:171: default role used (hint: for inline literals, use double backticks) (default-role)
sphinx-lint/tests/fixtures/friends/cpython/Doc/c-api/code.rst:171: default role used (hint: for inline literals, use double backticks) (default-role)

Also Git ignore coverage data files (for example: .coverage.Hugos-MacBook-Pro.local.95733.XMjioudx).