Generate RPC docs using regtest

Summary:
Using a mainnet bitcoind node to generate RPC docs is unnecessary,
especially since we plan to generate these docs on CI. ninja doc-rpc should
use regtest by default.

This patch adds support for passing arbitrary params to generate.go and uses it.

Test Plan:

bitcoind -regtest
ninja doc-rpc

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6971