Update developer docs to contain instructions for setting up new developer on GCP

[eecavanna]

These are instructions that someone with sufficient GCP permissions would follow in order to set up a new developer on GCP, so that person can work with the local GCS server.

I learned these instructions (with the help of @naglepuff and @pkalita-lbl) earlier today, and am creating this ticket to represent the task of documenting them.

I will test the document by following the instructions to set up access for @marySalvi.