Based of the README for now, with a small additional "welcome to the
docs" section.
Perhaps in future, we could re-use the README.md verbatim? Perhaps with
some additional post-processing.
I think we'd want to dramatically reduce its size first though.
By having a dedicated homepage outside of `user-guide`, this fixes#1625
Rather than nesting it as `config`, pass `makeNixvim`'s argument through
directly as a module.
This not only simplifies the implementation, but allows users to use
`makeNixvim` for any scenario where `makeNixvimWithModule` would normally
be required.
