[CI] Continuously update the man pages

Summary:
The automated man pages update is currently not fully automated, and is
triggered manually before each release. We can just update the man pages
as soon as they change so are always up-to-date.

Test Plan:

COMMIT_TYPE=update-manpages \
  ./contrib/source-control-tools/automated-commits.sh -p master

Repeat a few times and check the script skip committing as needed.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7269