User Profiles - Document disabled private profiles

What does this MR do and why?

Closes #464765 (closed)
Behind feature flag disallow_private_profiles

This change adds documentation for the admin and the user that explains the Disabled private profiles feature.

Screenshots or screen recordings

	Before	After
Admin Docs
User Docs

How to set up and validate locally

1. View markdown in review app and ensure it complies with our best practices

Related to #464765 (closed)