[docs] Dagster K8s deployment guide corrections - GH Issue #4234

Summary:
As raised in Issue 4234, the Helm deployment guides were a bit lacking. The very kind @mcallaway offered some great tips about improving it, so I incorporated those into the docs page.

Docs link at (incoming, may not have built yet): https://dagster-git-sid-issue-4234-elementl.vercel.app/master/deployment/guides/kubernetes/deploying-with-helm

Co-authored-by: Matt Callaway @mcallaway

Test Plan: Went through deployment guide, produced a successful run over S3.

Reviewers: rexledesma, johann

Reviewed By: johann

Differential Revision: https://dagster.phacility.com/D8841