feat: documentation webpage

### Is there an existing request for this?

- [X] I have searched the existing issues

### Feature Request

Write a documentation webpage for download, settings, setup and FAQ. MyST Markdown or mkdocs should be enough.

@awvwgk do you know if we would have to do anything more than adding a `docs/` dir and registering it with GitHub pages to get a URL under `fortran-lang.org`?