Docs: add note about VCS SSO in the manual import guide

[stsewd]

trafficstars

Our guide at https://docs.readthedocs.io/en/stable/guides/importing-private-repositories.html should warn users that if they have the VCS SSO integration enabled, they can't import projects manually, we could also add this warning in the import page itself, and block the action.

[humitos]

@agjohnson pinging you here since you have been working on this UX pattern on the new templates. Maybe you already solved this problem there and we can close the issue.

[agjohnson]

This will be addressed in the application with: https://github.com/readthedocs/ext-theme/pull/274

This PR is mentioning docs, which I do think should still be updated to cover these types of caveats more. The application UX is the primary place the users should encounter this information, but it's helpful to have supporting information in our docs too.