Rewrite GKE install guide with accurate Helm values and GCE internal ingress

- Simplify GKE cluster and Cloud SQL creation commands
- Replace Quickwit-style Helm values with standard CloudPrem chart format
- Add GCE internal ingress as a separate step (not yet supported in chart)
- Switch Datadog Agent setup to Datadog Operator (DatadogAgent CRD)
- Fix metastore secret name and storage config for GCS
- Remove outdated scaling and monitoring sections

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: fmassot