[GITIAN] Update scripts and docs to use the local gitian version

Summary:
This diff updates the docs to use the contrib/gitian-builder gitian
version. The documentation needs some more update but this is out of
scope for this patch.

Depends on D6973.

Test Plan: Run the CI gitian builds.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6976