Documentation process at GitLab

What does this MR do?

Introduces the process of shipping docs

To be discussed

• Do we want to be @mentioned?
• Do we want a "docs-follow-up" label to track the docs that weren't shipped within the same MR as the feature?

To be added

• Issue description templates
• MR description template not only for changing docs location but for new docs and doc updates
• Community contributions should include the docs (closes https://gitlab.com/gitlab-org/gitlab-ce/issues/46843)

Review apps

• Docs process: http://ce-docs-processes.178.62.207.141.xip.io/ce/development/documentation/workflow.html
• Docs structure: http://ce-docs-processes.178.62.207.141.xip.io/ce/development/documentation/structure.html