Add a link to RPC docs
This adds a link for users to navigate to the new RPC docs.
The link currently points to an nginx-generated index.
The index also contains a /doc/dev link, which is confusing, but that
will go away when we start generating correct index pages.
Depends on D7067
make container make run
Navigate to: Developers -> RPC Documentation
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7068