Improve documentation

[zawadz88]

Currently, the only documentation available is in Github README. There's also Github Pages with Dokka documentation and a sample app with most APIs covered, but it feels to me that the documentation should be better.
There were many new features added recently (https://github.com/zawadz88/MaterialPopupMenu#supported-features) but the documentation in README does not cover it at the moment.

A better, more organized documentation might decrease the initial learning curve and make finding available APIs easier. It should probably be moved to Github pages - I don't feel like we should clutter README too much.

Ideally, I would like to have README similar to this https://github.com/aalmiray/kordamp-gradle-plugins and wiki page generated by Asciidoctor like here: https://aalmiray.github.io/kordamp-gradle-plugins/