Do not trigger GitLab Pages pipelines any more

Do not trigger GitLab Pages pipelines any more

The bind.isc.org microsite no longer downloads documentation from
GitLab, instead linking to Read the Docs for all currently maintained
branches.  This makes it redundant to trigger GitLab Pages pipelines
after each merge to any of the maintained branches.  Revert changes
introduced by commit 31bde118db8cae4a0f5a9484e3beead66fcc24d0: remove
the "push:docs" job along with the "push" stage it is associated with
and revert artifact expiry time for the "docs" job to the default value.