Update admonition to show differences across plans (#93202)

As per https://github.com/grafana/support-escalations/issues/11312#issuecomment-2342024943, this PR updates the admonition to reflect that some features such as branding are not available for all plans.

docs/sources/setup-grafana/configure-grafana/configure-custom-branding/index.md

@@ -16,6 +16,10 @@ Custom branding enables you to replace the Grafana Labs brand and logo with your
 
 {{% admonition type="note" %}}
 Available in [Grafana Enterprise]({{< relref "../../../introduction/grafana-enterprise" >}}) and [Grafana Cloud](/docs/grafana-cloud). For Cloud Advanced and Enterprise customers, please provide custom elements and logos to our Support team. We will help you host your images and update your custom branding.
+
+This feature is not available for Grafana Free and Pro tiers.
+For more information on feature availability across plans, refer to our [feature comparison page](/docs/grafana-cloud/cost-management-and-billing/understand-grafana-cloud-features/)
+
 {{% /admonition %}}
 
 The `grafana.ini` file includes Grafana Enterprise custom branding. As with all configuration options, you can use environment variables to set custom branding.