Update doc dependencies in the doc itself

[ElectreAAS]

We changed themes a while back, but the hacking.rst file still mentionned the old one.

[emillon]

Correct but can we just point to doc/requirements.txt instead? It will contain the up to date deps, always.

[maiste]

@ElectreAAS is it good for you if we merge it?

[ElectreAAS]

Yes go ahead, I don't have the rights to do so myself