document usage of CAS as oauth2 / OIDC provider

[pmauduit]

As in the following PR on the gateway:
georchestra/georchestra-gateway#83

Also it might be of interest to document how to bypass the consent screen when selecting a oidc/oauth2 provider, see the bypassApprovalPrompt on the official doc

[pmauduit]

Also there are known bugs with the provided configuration (cas as oauth2 provider using the gateway, and as pure oidc provider it does not even work).

[pmauduit]

CAS as oauth2 is somehow 'documented' in the form of a docker composition here: https://github.com/georchestra/sample-docker-composition/tree/main/gateway/oauth2

And using a third-party OIDC here: https://github.com/georchestra/sample-docker-composition/tree/main/gateway/oidc

For now, configuring CAS as OIDC provider is complicated (needs to go through https, mismatch between localhost vs "real" / actual endpoint ...).