Update docs/architecture.md

[kzys]

Update and add more information on docs/architecture.md to reflect the current status.

https://matklad.github.io/2021/02/06/ARCHITECTURE.md.html is good guidelines about what we could put in the document.

I find ARCHITECTURE file to be a low-effort high-leverage way to bridge this gap. As the name suggest, this file should describe the high-level architecture of the project. Keep it short: every recurring contributor will have to read it. Additionally, the shorter it is, the less likely it will be invalidated by some future change. This is the main rule of thumb for ARCHITECTURE — specify only things which are unlikely to frequently change. Don’t try to keep it synchronized with code. Instead, revisit it a couple of times a year.