This changes the publishing process so that when publishing the guide
and the current version is a pre-release, it will be pushed to a
directory called `/pre-release/`.
This also switches from using simpleinfra's SSH-based script to a simple
push using normal git commands.
This switches to `gh` which is the more modern CLI, and also
available by default which removes the old installer script.
This also tightens the scope where GITHUB_TOKEN is exposed to just
the step where `gh` is executed.
Finally, it tightens the permissions on the GITHUB_TOKEN (though
`contents: write` is extremely permissive, since that allows writing to
almost anything in the repo).
