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