Add PSBT documentation

Summary:
Backport of Core PR13941, PR14319 and PR15012
The second and third PR are minor fixes to the new documentation page created in the first PR.

Test Plan:
Tried out the procedure described in Multisig with multiple Bitcoin ABC instances
Made sure all the RPC commands listed in the doc actually exist.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7758