update storage docs
The storage server documentation is out of date with our move to GCP. This MR updates the docs to be accurate for building new storage servers today. Additionally, it updates the doc to use "storage server" instead of NFS as hopefully we will be getting rid of NFS soon and "storage server" is a more accurate name.
Closes https://gitlab.com/gitlab-com/infrastructure/issues/4799
Edited by Alex Hanselka