Add build config for documentation, including RPC docs

Summary:
Building documentation should not rely on tests passing in some other
build. This patch splits out docs so they can be reliably built and adds
generating RPC docs to the mix.

Test Plan:

build-configurations.py build-docs
ls <build-dir>/artifacts/doc         # verify all doc files are generated correctly

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6979