Add Security Key setup guide and improve dark mode table styles

[ericbsd]

• Added comprehensive Security Key configuration guide covering FIDO2/WebAuthn setup, browser configuration, and troubleshooting.
• Enhanced dark mode CSS with specific table styles for better readability.
• Linked the new Security Key guide in the index file for easier navigation.

Summary by Sourcery

Add Security Key setup guide and enhance dark mode table styles

Enhancements:

• Improve dark mode table styling for better readability

Documentation:

• Add comprehensive Security Key configuration guide for FIDO2/WebAuthn setup
• Link the new Security Key guide in the main index navigation