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 31bde118: 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.

See #565 (closed)