8.9 missing docs

A meta issue to track all 8.9 features documentation. Open MRs for 8.9

Description Issue MR Docs?
.gitlab-ci.yml templates https://gitlab.com/gitlab-org/gitlab-ce/issues/17521 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4411 no
Environments https://gitlab.com/gitlab-org/gitlab-ce/issues/17009 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4605 yes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4725
Set label priority https://gitlab.com/gitlab-org/gitlab-ce/issues/14189 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4009 WIP https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3347
Request access to a project or group from GitLab UI https://gitlab.com/gitlab-org/gitlab-ce/issues/18584 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3798 yes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4685
Custom notification level https://gitlab.com/gitlab-org/gitlab-ce/issues/12758 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4389 yes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4389
Award emoji in comments https://gitlab.com/gitlab-org/gitlab-ce/issues/3655 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4291 no https://gitlab.com/gitlab-org/gitlab-ce/issues/9091
Artifacts expiry date https://gitlab.com/gitlab-org/gitlab-ce/issues/3439 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4200 partially https://gitlab.com/gitlab-org/gitlab-ce/issues/18694
Specific CI runners accessible outside of project https://gitlab.com/gitlab-org/gitlab-ce/issues/3407 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4093 yes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4093
Add Project Import/Export Functionality https://gitlab.com/gitlab-org/gitlab-ce/issues/3050 yes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4732
File Lock (EE option) https://gitlab.com/gitlab-org/gitlab-ee/issues/497 https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/440 yes https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/485
Allow a U2F Device to be the Second Factor for Authentication https://gitlab.com/gitlab-org/gitlab-ce/issues/15337 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3905 yes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3905
Manually create todos on issues & merge requests https://gitlab.com/gitlab-org/gitlab-ce/issues/15045 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4502 no https://gitlab.com/gitlab-org/gitlab-ce/issues/18923
Add option to restrict merge MR with failed build https://gitlab.com/gitlab-org/gitlab-ce/issues/5940 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4503 yes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4503
Artifacts for failed builds https://gitlab.com/gitlab-org/gitlab-ce/issues/4034 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4201 yes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4201
Add support for Docker Registry manifest v1 https://gitlab.com/gitlab-org/gitlab-ce/issues/18609 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4669 no
Fair usage of Shared Runners - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4634 no https://gitlab.com/gitlab-org/gitlab-ce/issues/18882
Allow experimentally to specify GIT_DEPTH and GIT_STRATEGY - https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/188 yes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4720
Add check_interval that allows to define in seconds how often to check GitLab for a new build - https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/198 yes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/198