The devguide's style guide says:

In the Python documentation, the use of sentence case in section titles is preferable, but consistency within a unit is more important than following this rule. If you add a section to a chapter where most sections are in title case, you can either convert all titles to sentence case or use the dominant style in the new section title.

The devguide has a wide range within units, many pages freely mixing both styles. You can see at https://devguide.python.org/#full-table-of-contents

I think the devguide, and at the very least its style guide, should set an example with sentence case.

📚 Documentation preview 📚: https://cpython-devguide--1162.org.readthedocs.build/