Add docs for Reaper Control Plane mode

[rzvoncek]

What this PR does: Adds docs on Reaper's control plane deployment.

Which issue(s) this PR fixes: Fixes

Checklist

• [ ] Changes manually tested
• [ ] Automated Tests added/updated
• [X] Documentation added/updated
• [X] CHANGELOG.md updated (not required for documentation PRs)
• [ ] CLA Signed: DataStax CLA