Change docs README for new way of building docs

Change docs README for new way of building docs

Docs used to be a separate repo who cloned pleroma and pelroma-fe.
Now the docs are just the BE docs and completely part of the Akkoma repo.
I moved back to using venv because that's what I used and cleaner imo since it keeps everything nice in the repo.
(Iirc virtualenv stored things in the Home folder or smthng)