adding name and namespace to existing rhdh cr example (#270)

related to final task on https://issues.redhat.com/browse/FLPATH-2457

## Summary by Sourcery

Documentation:
- Include 'name' and 'namespace' properties in the existing-rhdh Helm
chart example to clarify required CR configuration

Author: jenniferubah

docs/main/existing-rhdh.md

@@ -40,6 +40,8 @@ In 1.6, the Orchestrator infrastructure is installed using the Orchestrator Oper
         namespace: sonataflow-infra
       rhdh:
         installOperator: false
+        name: "my-rhdh" # Name of existing RHDH CR. Required
+        namespace: "rhdh" # Namespace of RHDH Instance, whether existing or to be installed. Required
     ```
     For users installing an existing-rhdh via the Helm Chart, note that the default values in the [Orchestrator CR under](https://github.com/rhdhorchestrator/orchestrator-go-operator/blob/main/config/samples/_v1alpha3_orchestrator.yaml) 'postgres' may need modifying.