Design doc to amend the work started in instructlab/instructlab#1022.

The current implementation is a good start. I see that ilab init takes a workspace path and when specified, it will ensure that the directory exists. I would like to get a more complete UX for the feature before we merge anything, though. I think it would be easiest to write out a design for the spec of the changes to CLI commands before implementing it. We can try doing that just in comments here, or if it gets too difficult, it can be a design doc posted to https://github.com/instructlab/dev-docs.

A starting point would be my original comment here: instructlab/instructlab#1022 (review)

To summarize - once a workspace is created and configured, I'd like to see everything done relative to that workspace instead of the current working directory. Otherwise, it feels incomplete and not enough of an improvement over mkdir workspace ; cd workspace.

Originally posted by @russellb in instructlab/instructlab#1022 (comment)

Also related: instructlab/instructlab#1034