docs: Add documentation for multiple SAML IdP support in GitLab Dedicated
What does this MR do and why?
Add documentation for multiple SAML IdP support in GitLab Dedicated
https://gitlab.com/groups/gitlab-com/gl-infra/gitlab-dedicated/-/epics/531 has added support for multiple SAML IdPs on GitLab Dedicated instances. This updates the public documentation to match the capability, including noting current constraints around Switchboard
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