Overhaul and introduce new type system documentation

[parlough]

Currently our type system docs are old and limited in scope.

We need to introduce new type system, type inference, and similar documentation, considering the state of the type system with null safety. These docs can potentially be included under Language > Types or a separate reference location. Some of these details can be gathered from Understanding null safety but that doesn't cover everything, it is written for readers transitioning from a pre null-safety world, and is perhaps too detailed for most users. We will need to find a balance.

This is related to #4603.

Edit: Much of this may belong in our reference style documentation (#4783).