docs: document WebAuthn conditional create

[StealthEyeLLC]

Description

Adds documentation for WebAuthn conditional create / automatic passkey creation.

Changes:

• Documents the mediation option on CredentialsContainer.create() and explains mediation: "conditional" for public key credential creation.
• Adds a create() example showing feature detection with PublicKeyCredential.getClientCapabilities() and a conditional passkey creation request.
• Adds an Automatic passkey creation section to the Web Authentication API overview.
• Clarifies that the conditionalCreate client capability refers to automatic passkey creation, not just creating discoverable credentials.

Motivation

Conditional create lets sites request passkey creation after a successful password sign-in, helping users adopt passkeys without needing to explicitly find and click a separate “create passkey” control.

Fixes #42516.

Additional details

Validated with:

• git diff --check
• markdownlint-cli2 on the changed files using Node 22.23.1
• prettier -c on the changed files using Node 22.23.1