Improve documentation

[AliSoftware]

• Review all existing documentation to ensure it's still valid even after the Great Split and deprecated Context variables and all
• Make 👌 documentation (for the global structure of SwiftGen, better README for each repo, doc for templates, …)

[AliSoftware]

See SwiftGen/SwiftGen#232 (comment) for the ASCII-art diagram documenting the overall structure of SwiftGen ecosystem.

A real PNG drawing could be better and more responsive too though, to be readable both on Mac and phones (in that case let's keep the source of the PNG — like the .graffle document — in this repo too to be able to modify it later)