Document architecture of the core monorepo

If you're new to the monorepo, you might want to know:

- How is the monorepo structured?
- Where are all of the packages located?
- What is the general shape of a package?
- What do all of the "other" files in the monorepo do and how are those laid out? (Hint: most of these are configuration files, but for some tools, there are two levels of configuration.)

We should talk about these things in the Contributing document.

Start by reading the Contributing document and surveying the monorepo structure, package locations, package layout, and configuration files. Document the answers to the four questions in that document, including how tools with multiple configuration levels are organized.

Written by the indexing model from the issue text.