Update Releases documentation

Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23901.

Update GitLab's Releases documentation:

  • Crosslink from project/index.md
  • Move project/releases.md into project/releases/index.md, as it's likely to be a big feature and will need more docs under releases/

Checklists

  • Add the correct labels
  • Add the correct milestone
Edited by Marcia Ramos