Merge #14530: Use RPCHelpMan to generate RPC doc strings

Summary:
Backport PR14530

Pull request description:

This introduces a manager for the RPC help generation and demonstrates its use of it in some RPCs.
...

Backport note:
This change is nice when backporting additional RPC, so we can grab the
RPCHelpMan stuff and keep it in.

Some changes were made as we don't have replaceability or smart fees, otherwise
pretty much just lint differences.

Test Plan: ninja check-all ; read carefully

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5239