docs: consolidate UPGRADE_GUIDE.md and MIGRATION_GUIDE.md (#109)

Merged MIGRATION_GUIDE.md into UPGRADE_GUIDE.md to reduce documentation
fragmentation and improve discoverability.

Changes:
- Renamed to "Upgrade & Migration Guide" (v2.0)
- Added "Security Migration (AppRole & TLS)" section covering:
  - Migration scenarios table
  - Root Token → AppRole migration steps
  - TLS dual-mode and TLS-only setup
  - Adding AppRole to custom services
  - Security migration rollback procedures
- Added FAQ section covering version upgrades, AppRole/TLS, and profiles
- Deleted MIGRATION_GUIDE.md (content now in UPGRADE_GUIDE.md)
- Updated docs/README.md index
- Updated ROLLBACK_PROCEDURES.md cross-references

Author: NormB