Deploy leptonic.dev for all versions of the library

[lpotthast]

The website always shows guides / documentation for the latest version of Leptonic. To be concrete, it currently shows the current master state.

This does not allow users stuck on older versions to view guides related to the version they are using.
It also doe not reflect the latest release. As most people should use a release and not depend on master, this is an issue.

The version info in the top right should be made into a version selector. Each version of the website needs its own deployment, based on the git tag of that version. Versions of the website must be available at paths like 'leptonic.dev/v0.5'.

The latest release should be the default version. Viewing guides for an upcoming release (current master branch state) should still be possible.