Restructure and ARCHITECTURE.md

[daniel-vainsencher]

We should break up the single big file, and document in an ARCHITECTURE.md files as in https://matklad.github.io/2021/02/06/ARCHITECTURE.md.html or something along those files.

@fox-daniel as we discussed. Its not obvious this is a top priority for either of us, so whoever starts working on this should create a WIP PR and link here to avoid a race.