docs: describe extension support

[shaloo]

• Have you updated CHANGELOG.md with relevant non-documentation file changes?
• Have you updated the documentation for this change?

---

[jahn-junior]

The spell check failure is caused by a bug that came about when we incorporated sphinx-llm. I'm bringing in the patch fix from the sphinx-stack repo with #17.

[shaloo]

@jahn-junior would you know by when you'll be able to bring in the patch fix here for the spell check failure resolution?

[SecondSkoll]

If you rebase this now @shaloo it should pass.

[a-velasco]

Thanks for the bonus whitespace cleanup :)

[jahn-junior]

The added text doesn't quite reflect our supported extensions. We do support all of the extensions listed in the Sphinx Stack requirements.txt file. Because canonical-sphinx is included in this file, we do also support the extensions it sets up.

However, we do not recommend using the optional full dependency group, and this change could be interpreted as though we do. So, I think for simplicity's sake, we should just say that we only fully support the extensions listed in the default requirements.txt file. Since canonical-sphinx is getting deprecated soon, this is also more future-proof.