docs: document emergency withdraw timelock and nonce cancellation

[vku2018]

Closes #1725.

Summary

• add the requested top-level docs/EMERGENCY_WITHDRAW.md operator guide
• document the initiate/monitor/execute/cancel flow, timelock boundaries, nonce cancellation, reserve-safety checks, events, and operator checklist
• link the new guide from the README documentation list

Validation

g -n "initiate_emergency_withdraw|execute_emergency_withdraw|cancel_emergency_withdraw|get_pending_emergency_withdraw|can_exec_emergency|emg_time_until_unlock|emg_time_until_expire|Nonce Cancellation|DEFAULT_EMERGENCY_TIMELOCK_SECS|EmergencyWithdrawExpired" docs/EMERGENCY_WITHDRAW.md README.md

• git diff --check
• cargo --version is blocked locally because Cargo is not installed in this environment