Document how Omni is more secure than native Talos

[steverfrancis]

Omni adds several layers of security over Talos, but this is not to be found in the docs.
(CLuster secrets never exposed; more secure node joining; integration with IDP, etc)

[rothgar]

All of those topics are covered in the docs on various pages, but not from a single page.

Siderolink, join tokens, IDP, and secrets are all discussed on pages related to each of those features. A combined overview of how they tie together to provide a more complete secure foundation vs Talos should be a marketing landing page on siderolabs.com