OAuth2 client credential information does not cover "external projects"

The recommendation to use Lookup groups to manage projects outlined in https://docs.raven.cam.ac.uk/en/latest/creating-oauth2-client-credentials/#create-a-google-project only works for projects created within the cam.ac.uk organisation and not external projects.

Update the documentation to make this distinction clear. Until we have a formal procedure for requesting Google projects, recommend that external projects use a shared admin account.