doc: establish guidelines on how users should migrate their content for each provider over and test end-to-end to avoid mishaps.

[justin13888]

• Prerequisite: Stabilize exporters for Google Photos, iCloud Photos, Immich
• Point to instructions on how to transfer data+metadata for each
• Point to documentation on
• Outline disclaimers about software robustness. (We take data seriously but it's only as strong as the weakest chain. Users should transfer, verify everything is there end-to-end, use it for a few months, and follow all deployment best-practices to minimize catastrophic errors.)