Move documentation to a more central location
Right now the documentation is in multiple locations, can be hard to find, and it doesn't appear very official:
- The main docs are at https://pygobject.readthedocs.io. This is very common for the Python ecosystem to use RTD.
- The latest API docs are at https://amolenaar.pages.gitlab.gnome.org/pygobject-docs/
- The PyGObject Guide is at https://rafaelmardojai.pages.gitlab.gnome.org/pygobject-guide/
I recommend we get a URL at https://gnome.pages.gitlab.gnome.org/pygobject/ that could contain all these resources.