Document architecture of the monorepo

[mcmire]

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.

[xinnanyemm]

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