Documentation for `joinmarket.cfg`

The full `joinmarket.cfg` config file contains many settings and multiple sections (daemon, blockchain, messaging, logging, timeout, policy, payjoin, yield generator), not all of which are immediately relevant to most users.

We should understand and document all settings, and break them down into two or three different sections, e.g. basic / advanced / expert.

Rough tasks list:

- [x] Research `joinmarket.cfg`
- [ ] Rank-order by importance and/or use-case
- [ ] Write copy, find proper names and descriptions for settings (in cooperation with @GBKS)
- [ ] Update the [JM docs](https://github.com/JoinMarket-Org/joinmarket-clientserver/tree/master/docs) upstream

All of the above should serve as a base for implementing #12