docs: show newest release, consider versioning

[jstriebel]

At the moment, the docs reflect the current master state. However, users usually want to see the version that is currently available via PyPI. Since they might not use the newest version, having the docs versioned themselves might be useful. This might be quite simple using the builtin support in mkdocs-material and supplying a custom versions.json (see here for the docs and here for and example). However (if it's not that straight-forward), it's most important to show the current release first instead of the master.