Add source hook helper docs and other docs fixes

[mprpic]

This also fixes a couple unrendered examples in https://fromager.readthedocs.io/en/latest/customization.html#combining-both-settings.

[LalatenduMohanty]

The PR looks good to me, however there 5 commits which I think can be merged to couple of commits.

1st commit

docs(hooks): add source module helper functions documentation

- Document ensure_pkg_info, pep517_build_sdist, and unpack_source
- Improve docstrings in sources.py for autodoc

2nd commit

docs: improve documentation structure and rendering

- Fix literalinclude rendering in customization.md with MyST colon_fence
- Move skip-constraints example to multiple-versions how-to guide
- Update conf.py: fix typos, use dynamic copyright year, add MyST extension