Adds documentation website using https://vitepress.dev.
Some decisions are questionable, for example creation of a symbolic link
for the README in the `docs/`. I wanted to preserve the ability to read
docs using Github. Maybe not needed.
Another issue, is the page outline for the `NixOS` chapter is too
narrow.
But should serve as a scaffold/basis for doing it properly.
Adds a new Nix flake output `headplane-nixos-docs` which generates a
Markdown page with all NixOS `services.headplane.settings.*` options.
Replaces existing handwritten options documentation.
Aarnav Tale