Use different colour for 'Return value: Borrowed reference'

[hugovk]

Follow on from python/cpython#117926.

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.

This PR uses the same yellow/orange colour as the "Changed in version x.y" text in #185 - I've already checked the contrast ratios in that PR and both meet WCAG AAA. By using the same colour, it will be easier to check/change when we audit.

For example: https://python-docs-theme-previews--188.org.readthedocs.build/en/188/c-api/exceptions.html#querying-the-error-indicator