docs: add helm doc
Created by: michaellzc
"dark launch" helm docs
extracted from https://github.com/sourcegraph/sourcegraph/pull/32463
this PR focuses on enabling prospects to deploy Sourcegraph on one of the major cloud providers. I try to include the right amount of detail (I think) without being too specific that we will have a hard time keeping up with upstream changes.
it pretty much translates the "Configure a storage class" and "Configure network access" sections from https://docs.sourcegraph.com/admin/install/kubernetes/configure and combine them together (group by cloud vendor instead of topics).
In future PR, we should try to add more advanced configuration, e.g. using external databases
Notes
Some referenced files won't be available until below PR is merged
Test plan
It docs