Add RPC commands indices, by version

Summary:
This patch adds a generator plugin that scans existing RPC doc markdown files and generates an index for each version of ABC.

Depends on D7129

Test Plan:

make serve

Navigate to http://localhost:4000/doc/0.21.12/rpc/

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7130