Adds page up/down scrolling support to respectively scroll up/down by a full page.
Note that this is mostly just to get the feature out for those interested, and is admittedly a bit rushed - I will be rewriting all logic involving event handling as part of state refactor anyways, so this will also get changed in the work done there, and therefore, I kinda just sped through this.
A large migration of documentation over to mkdocs, and some rewrites. Some stuff (install information, basic supported systems, contributors, thanks) are still staying in README.md, and CONTRIBUTING.md is essentially duplicated right now. However, stuff like configuration and key/mouse bindings are now moved to mkdocs.
Some parts are still a bit WIP - it is definitely not done (documentation never seems to be...). However, it should be "good enough" for now, and I'm much happier working with the documentation in this form than trying to scroll through a giant endless README.md file. It also works much better for adding new documentation.
