doc: Add external interface consistency guarantees

Summary:
This adds a new documentation page about the JSON-RPC Interface
Backport of Core PR14592

Test Plan: Check markdown formatting in a markdown viewer.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7765