docs: map the Find My network integration contract

[IvanChanPing]

Summary

• records the complete Apple-account/APS/CloudKit/keychain prerequisites for network location
• maps BeaconStore sync, rotating-key derivation, report fetch/decryption, and state persistence in order
• defines the narrow Rust-to-Android API and credential-storage boundary

This is an implementation-contract draft for #717, not a completed Find My client. It intentionally avoids substituting the incomplete Kotlin tracker prototype for OpenBubbles' full authenticated stack.

Verification

• cross-checked against OpenBubbles rustpush account setup and FindMyClient::sync_item_positions source
• documentation diff checked locally