Documentation updates #13681

Since we're starting to plan a 1.0 release, I'm taking the opportunity to do a pass over the mypy docs.

Some overarching goals include:

• Increase consistency. It's pretty common to see multiple places in the mypy docs where we talk about the same thing in different ways.
• In particular, the command line and config file reference often diverge in ways that are surprising to users and contributors (we should maybe unify these entirely)
• Fix up out of date references. In various places, things are slightly out of date in 2022.
• Make changes with a global view of the docs. Various sections have accreted information over time. Splitting out, reordering and relinking things can make things more accessible to newcomers to the type system and lend a sense of progression as you read through the docs.
• Make things more accessible. The mypy docs are the best Python type system reference out there, so it's important to make sure we continue to serve newcomers well

I'm not planning on proactively explaining all my changes in each PR, but if you're reviewing them, please feel free to ask why I changed something.