docs: add passkey registration security documentation #396

Job	Run time
Cargo Package Verify	1m 39s
Docker Context Guard	1m 41s
Marketplace Packer Validate	7s
Build (ubuntu-latest)	1m 41s
Build (macos-latest)	1m 41s
Format Check	13s
CLI Parity Tests	1s
	7m 3s

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