PR Summary

Update the "Writing documentation" for the changes from#25395 to point out the possibility of galleries with *.py and *.rst files in them (with links to the SG documentation about this).

Also added a table of galleries with their sources and where they end up in the build.

PR Checklist

Documentation and Tests

• Has pytest style unit tests (andpytest passes)
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a.. versionadded:: directive in the docstring and documented indoc/users/next_whats_new/
• API changes are marked with a.. versionchanged:: directive in the docstring and documented indoc/api/next_api_changes/
• Release notes conform with instructions innext_whats_new/README.rst ornext_api_changes/README.rst