docs: explain how to use Shorebird in a headless environment (e.g. IDX)

[eseidel]

Some users would like to use Shorebird in a headless environment.

Should be possible with shorebird login:ci or otherwise copying ones oauth token from a local machine to the headless machine.

We could write some docs explaining such other than:
https://docs.shorebird.dev/ci/generic/

[AruArshad]

Works fine. Thanks.