High level guide is heavily outdated in the technical details

[AdamISZ]

This document: https://github.com/JoinMarket-Org/JoinMarket-Docs/blame/1e9aabab9e459edd3ab4764ec270304859db3bae/High-level-design.md is now way out of date in its description of, and links to, the code. For example, the way it describes the Wallet object is all wrong since around 2017, and same for the Notify thread (we don't even use threads any more).

A first PR might just remove all the tech/code stuff and leave in the architecture.