Align SKDM flow docs with forward-secrecy and cache hygiene fix

[mintlify]

Summary

Updates the docs to match oxidezap/whatsapp-rust#604 (fix(send): close SKDM flow gaps for forward secrecy and cache hygiene).

What changed

• Per-device cache invalidation table (advanced/signal-protocol.mdx): replaced "invalidate all entries" rows for patch_device_add/patch_device_remove with the new behavior — adds rely on device_has_key() returning None for unseen ids; removes delete only the matching sender_key_devices rows and evict only cache entries indexing (user, device_id). Added rows for phash mismatch (now also clears persisted state with own-key fallback), periodic rotation at chain iteration ≥ 1000, and participant-remove forward-secrecy rotation.
• New sections in advanced/signal-protocol.mdx:
  • "Forward-secrecy rotation on participant removal" — describes rotate_sender_key_on_participant_remove.
  • "Periodic sender key rotation" — describes the chain-iteration ≥ 1000 trigger.
• DeviceListChanged event (concepts/events.mdx): rewrote the SKDM paragraph to reflect that no global cache wipe happens on device add/remove.
• ProtocolStore trait (api/store.mdx, concepts/storage.mdx, guides/custom-backends.mdx): documented the new delete_sender_key_device_rows method and added a chunked SQLite example for custom backends.
• Phash sections (api/send.mdx, api/status.mdx, advanced/signal-protocol.mdx): clarified that group/status mismatches now clear persisted sender_key_devices rows (with an own-key delete fallback if the DB clear fails) and group sends now also invalidate the group info cache on mismatch.