fix: Update RTD config.

[dianakhuang]

RTD updated their config format. This updates the cookiecutter to match their recommendations.

https://github.com/openedx/edx-cookiecutters/issues/416

Merge checklist: Check off if complete or not applicable:

• [x] Changelog record added
• [ ] Documentation updated (not only docstrings)
• [x] Fixup commits are squashed away
• [x] Unit tests added/updated
• [x] Manual testing instructions provided
• [x] Noted any: Concerns, dependencies, deadlines, tickets

[dianakhuang]

Closing as a dupe of https://github.com/openedx/edx-cookiecutters/pull/419