docs: migrate a chunk of documentation over to mkdocs

[ClementTsang]

Description

A description of the change and what it does. If relevant (such as any change that modifies the UI), please provide screenshots of the change:

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.

Issue

If applicable, what issue does this address?

Closes: #383

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README, help menu, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts