Move untested scenarios 2 and 3 to experimental doc

Separated the tested Scenario 1 from the untested Scenarios 2 and 3
to keep the main documentation focused on production-ready procedures.

Changes:
- Created new backup-restore-ctlplane-experimental.md for untested scenarios
- Moved Scenario 2 (Different Namespace) to experimental doc
- Moved Scenario 3 (Different Cluster) to experimental doc
- Added reference in main doc pointing to experimental scenarios
- Clear warning that only Scenario 1 has been tested

Main doc now focuses exclusively on the tested restore procedure.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: stuggi