Add documentation for `TypedDict` return values of methods usable by 3rd parties

### What was wrong?

While making an application that uses the web3.py library I found myself really wishing there were strong return types for the various methods like `w3.eth.get_block(...)`. Basically, I want to have something like a `TypedDict` which strongly defines the return types of the various web3 methods.

### How can it be fixed?

The `web3` library should have `TypedDict` types for all of the various return types that return a mapping. These types should be importable by 3rd party libraries (probably via `web3.typing`) and should also be documented (probably in a new section of the documentation focused on how to do typing for 3rd party applications).