Support for generated files

Add support for including generated markdown docs, like NixOS option docs. Maybe just extraCommands or so, which are then added to the shell script for building and running?

Watch makes it harder, as it uses the real docs directory. Maybe just have existing files with placeholders which just get replaced in the real build step? Like https://github.com/nix-community/nixvim/blob/main/docs%2Fmdbook%2Fdefault.nix#L362