Security guides

This first batch publishes 4 of the 17 upstream Security guides; the rest are converted and will be released in later batches.

• Passkey Authentication — Authenticate a returning user with a passkey for primary sign-in.
• Passkey Registration — Register a passkey for an existing user account.
• Prevent DOM-based XSS attacks with Trusted Types — Prevent DOM-based XSS attacks by defining a Trusted Types policy to sanitize untrusted HTML, using the policy when writing to DOM sinks like innerHTML, and enforcing Trusted Types via Content Security Policy.
• Sanitizing Untrusted HTML — Safely parse and display untrusted HTML content from user input by removing unsafe elements and attributes.