rework doc about oidc-agent usage

we've seen that generating an account for the oidc-agent on a linux machine without $DISPLAY set, the agent assumes that the device-code-flow is necessary and fails because it is not supported (by choice?) with login.helmholtz.de. a hint should be added to the documentation that setting the display variable with export DISPLAY=0: mitigates this issue and oidc-gen --pub --no-url-call --no-webserver works as intended afterwards. oidc-agent then outputs a URL that can be copied to a browser and used for login.

the user data displayed in the doc still refers to carsten heidmann and could be changed such that "no name" is shown instead.