docs: Add initial index.md landing page
This is the page we want to link to users wanting to get started with developing GNOME Settings. It should contain the pointers users need to find their way around contributing to the project.
Notice that gitlab already renders it by default in the /docs folder https://gitlab.gnome.org/GNOME/gnome-control-center/-/tree/ecb60a98f23a479ea5359de76ecb74b207629add/docs
This could later be generated/managed by a little publishing/dynamic script.