Add literal formatting for extlinks and get rid of custom extension

[SilverRainZ]

Hi python devguide maintainers,

I wrote an extension that can compose multiple reStructuredText roles together. I learned from https://github.com/sphinx-doc/sphinx/issues/11745#issuecomment-1888465545 that devguide might need this feature, I hope this PR can help.

---

📚 Documentation preview 📚: https://cpython-devguide--1298.org.readthedocs.build/

[ghost]

All commit authors signed the Contributor License Agreement.

[AA-Turner]

Obviated by #1351.

A