Help admins understand which image to customize for different purposes

[kmasiello]

It's a lot to expect an admin to appreciate the interplay between workbench, workbench-session-init, workbench-positron-init, and workbench-session.

Provide a quick reference to direct admins to which image to customize for different use cases:

e.g.,

1. To add additional system dependencies or versions of R/Py/Qu: customize workbench-session using examples as a guide
2. To update the version of Workbench, update workbench and workbench-session-init
3. To change workbench config, customize workbench
4. To install a more recent positron version than what ships with workbench, use workbench-positron-init
5. etc.