Docs: Releases permissions

What does this MR do?

Documents project's Releases permissions.

Related issues

• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56471.
• Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/56738#note_136028273.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from other subject-related docs

Review checklist

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