Add Comparison to Google Cloud Shell

## what
* Geodesic has some shared concepts to the Google Cloud Shell

## why
* It might help to explain that `geodesic` can be used as a "cloud shell", although it does much more than that.

## references
* https://cloud.google.com/shell/

No documentation file or test is named in the issue. Read the Geodesic documentation structure and compare its shared concepts with the Google Cloud Shell reference at cloud.google.com/shell; done means adding a clear comparison that explains Geodesic's cloud-shell use and its broader capabilities.

Written by the indexing model from the issue text.