When submitting a phabricator diff to the swh-docs repository, a Jenkins
job will be executed in order to check sane build of the full Software
Heritage documentations (dev, user, sysadmin).

Herald rule and Harbomaster configuration to launch the job have been
setup on the Phabricator side.

Jenkins job has been manually configured for testing,
the new job configuration in that diff will automatically recreate it.

See D6519 as test diff where the CI job has been executed.

Closes T3661