update docs to new joinmarket-ng

[MarnixCroes]

The docs are still using/referencing to https://github.com/JoinMarket-Org/joinmarket-clientserver, which has become history

Use the new https://github.com/joinmarket-ng/joinmarket-ng

[theborakompanioni]

Some very good deep dive info available here: https://github.com/joinmarket-ng/joinmarket-ng/tree/main/docs/technical