Version selector in documentation

[michallepicki]

When packages get published to Hex, there is a special docs_config.js file available for documentation (generated by hex backend) with a list of all available versions of that package (example). It can be used to show a versions selector on documentation pages.

I worked on that feature for ex_doc and for gleam. The gleam implementation should be pretty easy to copy, may just need a few visual tweaks.