Add sub navigation for RPC documentation

Summary: This makes it easier to navigate between RPC commands.

Test Plan:
make serve
Navigate to http://localhost:4000/doc/0.21.12/ and navigate to and between various commands.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7205