Show dev / `main`-branch docs too

[huonw]

Per #155 (comment), docs from the main branch are rendered, but hidden from the version selector dropdown to reduce the noise/keep it streamlined.

E.g. https://www.pantsbuild.org/2.20/docs/introduction/welcome-to-pants is available but not visible in the dropdown:

This was discussed briefly on slack and there seemed to be a bit of a consensus that they should be shown. This would help, for instance, reviewing/validating the docs and people doing pre-release testing of new features.

[huonw]

I've adjusted how the versions are configured, in #176, but won't finish this off now.