[DOCS-13885] Note that Cisco Duo Federal is not supported

[evazorro]

What does this PR do? What is the motivation?

Fixes DOCS-13885

The Cisco Duo integration supports standard Duo accounts (duosecurity.com domains) but does not support Duo Federal accounts (duofederal.com domains). This PR adds a note at the top of the Setup section to make that limitation clear to readers.

The note only appears if you have site=US1-FED selected, and looks like this:

[dd-octo-sts]

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