antsibull-docs should expose optional ansible dependencies inside an extra

[ssbarnea]

I spotted few dependencies that are not listed in any extra, and this might impose extra maintenance for users of anstibul-docs if these are not updated in-sync.

ansible-pygments
sphinx
sphinx-ansible-theme >= 0.9.0

So, instead of listing them separated in our projects, it would be much easier to just install antsibull-docs[ansible] and forget about them getting out of sync.

[felixfontein]

Note that sphinx is already a direct dependency of antsibull-docs.