Add an overview page to show available versions for the API docs

[straight-shoota]

Currently the website doesn't directly show which versions are available in the API docs. There's the version selector since crystal-lang/crystal#9074 on each individual API docs page. But this only works with JavaScript and is somewhat limited in capabilities.

Ideally such an overview should show more information than just version names and link to API docs of that version.
Basic information is already available in a machine-readable format at https://crystal-lang.org/api/versions.json
This could be further enhanced by relevant data, but a basic list would already be an improvement.

I'd like to add a noscript alternative to the version selector, which should contain of a link to a versions overview page (cf. crystal-lang/crystal#7011 (comment)).

[straight-shoota]

Resolved by #215 and #416