APE 25ish: An initial roadmap for static type checking

[namurphy]

This PR will add an APE that proposes a path forward for enabling static type checking in Astropy's continuous integration suite. If anyone is interested in being a co-author, please let me know!

The process would be more or less to:

• Enable mypy in CI, but only check a single file at first
• Gradually enable static type checking in one or two subpackages (astropy.cosmology and astropy.units?)
• Add a section to the developer docs on type hint annotations and static type checking
• Allow subpackage maintainers to gradually enable static type checking in more subpackages

It's currently in an early draft state, and might be best though of as being in the brainstorming stage 🧠 🌩️. Big picture feedback would be especially appreciated! In particular, we will need to address the really valid concerns and considerations that have been brought up in https://github.com/astropy/astropy/issues/15170.

I'm also hoping to avoid being over-prescriptive, so as to give us flexibility and avoid the need to amend this APE when ruff implements a full static type checker. 🙃

Closes #92.

[namurphy]

It's going to be a while before I can get back to this PR, so if anyone else wants to work on this, please let me know!

My main goal here has been to encapsulate the advice given in the mypy docs on enabling static type checking on an existing code base.