docs: update observability-plane URLs for non-localhost defaults

The observability-plane chart no longer ships *.localhost defaults for the
control-plane API URLs (openchoreo/openchoreo#3845) and introduces a dedicated
observer.publicUrl value (openchoreo/openchoreo#3846).

- Helm values reference: controlPlaneApiUrl / openchoreoApiUrl now show the
  .invalid placeholder default; document observer.publicUrl.
- Bring-your-own-environment guide: set observer.controlPlaneApiUrl (so the
  initial install passes hostname validation) and observer.publicUrl.

Signed-off-by: Rashad Sirajudeen <rashad@wso2.com>

Author: rashadism

docs/getting-started/try-it-out/on-your-environment.mdx

@@ -945,6 +945,7 @@ OpenChoreo uses a modular observability plane. In this step you will:
 observer:
     openSearchSecretName: opensearch-admin-credentials
     secretName: observer-secret
+    controlPlaneApiUrl: "https://api.\${CP_BASE_DOMAIN}"
 gateway:
     tls:
       enabled: false
@@ -1041,6 +1042,7 @@ observer:
     openSearchSecretName: opensearch-admin-credentials
     secretName: observer-secret
     controlPlaneApiUrl: "https://api.\${CP_BASE_DOMAIN}"
+    publicUrl: "https://observer.\${OBS_BASE_DOMAIN}"
     http:
       hostnames:
         - "observer.\${OBS_BASE_DOMAIN}"