[CMAKE] Make the doc-rpc target actually generate the rpc docs

Summary:
The current target runs the go script, which is not enough to generate
the docs. It also requires a bitcoind instance running on regtest, and
the user has to go through the logic of starting the server, running the
target and shutting down the server.
This diff scripts this process so that the doc-rpc target handles all
of this and actually generates the RPC docs.

Test Plan:

ninja doc-rpc

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7409