[9.18] chg: ci: Check that generated changelog entry doesn't break docs build

Since changelog entries are now generated from MR title&description,
they aren't sanity checked during a regular docs build. If these contain
special sequences that will be interpreted by sphinx, it might result in
breakage that would have to be amended manually.

Add a CI check to test a doc build with changelog after the MR is merged
to ensure that the docs can be built when generating changelog from
pristine git contents.

Related #4847

Backport of MR !9294

Merge branch 'backport-nicki/add-changelog-entry-check-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9312