added Azure AD Certificate call out

[billmcilhargey]

Azure Ad Certificate based MFA is in preview and based on this document, it only works on Home Tenants (if I read this right)

https://learn.microsoft.com/en-us/azure/active-directory/authentication/concept-certificate-based-authentication

Specifically:

"External identity support An external identity can't perform multifactor authentication (MFA) to the resource tenant with Azure AD CBA. Instead, have the user perform MFA using CBA in the home tenant, and set up cross tenant settings for the resource tenant to trust MFA from the home tenant.

For more information about how to enable Trust multi-factor authentication from Azure AD tenants, see Configure B2B collaboration cross-tenant access." https://learn.microsoft.com/en-us/azure/active-directory/authentication/concept-certificate-based-authentication-technical-deep-dive#external-identity-support

[prmerger-automator[bot]]

@computeronix : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[opbld31]

Learn Build status updates of commit 9fc60fa:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
articles/active-directory/external-identities/authentication-conditional-access.md	:white_check_mark:Succeeded

For more details, please refer to the build report.

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[Jak-MS]

@msmimart

• Can you review this PR?
• IMPORTANT: When this content is ready to merge, you must add #sign-off in a comment or the approval may get overlooked.

#label:"aq-pr-triaged" @MicrosoftDocs/public-repo-pr-review-team

[msmimart]

@namkedia - Can you please confirm?

[msmimart]

Thanks again for your contribution. However, we're currently evaluating this content and will update the docs as needed; please stay tuned. Your feedback is much appreciated! #please-close