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

• &403 (closed)

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/8860

Author's checklist

• Apply the correct labels and milestone

Review checklist

• PM's review (recommended, but not a blocker)
• Technical writer's review (required)