Currently light green is used for all return types

• "Return value: Always NULL."
• "Return value: New reference."
• "Return value: Borrowed reference."

For example:https://docs.python.org/3/c-api/exceptions.html

A borrowed reference is not necessarily a "bad" thing, but we may want to use something other than green that indicates it's "good", to indicate they should be treated with care.

One option is yellow.

1. These annotations are added inDoc/tools/extensions/c_annotations.py. The first step is to add CSS classes in this file.

2. The light green colour is defined inhttps://github.com/python/python-docs-theme. The second step is to adjust the colours in the theme.

This PR also includes some light cleanup whilst we're touching this file, see commits.

📚 Documentation preview 📚:https://cpython-previews--117926.org.readthedocs.build/