[docs] project structure

[Rich-Harris]

This is a shorter/simpler version of #4829

Closes #4829
Closes #4748

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[benmccann]

lgtm

the choice of which files to include seems difficult. e.g. if we include tsconfig.json should we also include src/app.d.ts? I don't really know. what's here seems fine to me though

[Rich-Harris]

It's a little tricky, yeah. We can iterate on it over time. The main reason I included tsconfig.json is that we need to explain the whole extends thing. I wouldn't be opposed to adding src/app.d.ts in a follow-up