BD-3330: Add FAQ for field-level encryption

[lydia-xie]

Summary

My PR...

Related PRs, issues, or features (optional)

• Closes BD-3330

Feature release date (optional)

• N/A

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Thanks for contributing! We look forward to reading your work.

[tiffanyjlin]

Suggested response (feel free to reword):

No. Mail-to list-unsubscribe would send the plaintext decrypted email address to Braze. With field-level encryption enabled, we support the URL-based http: method, including one-click. We also recommend including a one-click unsubscribe link in the email body.

[rachel-feinberg]

Lgtm! Left a small suggestion