Remove stuff about models of type theory

Add references to specific (noteable) implementaitons of category theory:
* Unimath
* cubicaltt
* https://github.com/pcapriotti/agda-categories
* https://github.com/copumpkin/categories
* ...

Talk about structure of library:
===

Propositional- and non-propositional stuff split up
Providing "equiality principles"
Provide overview of what has been proven.

What can I say about reusability?

Misc
====

Propositional content