Document conditional WebAuthn credential creation

[tahakotil]

Description

Documents conditional credential creation (automatic passkey creation) across the relevant WebAuthn pages:

• adds the mediation option, rejection behavior, and a conditional creation example to CredentialsContainer.create()
• separates automatic passkey creation from authentication autofill in the Web Authentication API overview
• clarifies what the conditionalCreate client capability represents

The behavior and terminology follow the Credential Management specification change in w3c/webappsec-credential-management#224.

Motivation

Developers could discover conditionalCreate in getClientCapabilities(), but MDN did not explain how to use mediation: "conditional" with navigator.credentials.create() or how that behavior differs from conditional get()/autofill.

Additional details

• Markdownlint: 0 issues across the three changed pages
• Prettier: all three pages pass
• CSpell: 0 issues across the three changed pages
• Rari build with warnings denied: 3 pages built, no issues found

Fixes #42516