Mention how to promote an issue to an epic
What does this MR do?
While we have docs for promoting issues to epics, we don't mention how to do this. I also added more info what promoting entails.
Related issues
Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/8860
Author's checklist
Review checklist
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PM's review (recommended, but not a blocker) -
Technical writer's review (required)
Edited by 🤖 GitLab Bot 🤖