Add install of helm-util to docs

* Update helm.mdx

For clarity, include the `jb install` command for those following the example.

* One more sentence.

Co-authored-by: malcolmholmes <42545407+malcolmholmes@users.noreply.github.com>

docs/docs/helm.mdx

@@ -18,7 +18,11 @@ resource definition, you can still consume Helm resources, as described below.
 
 Helm support is provided using the
 [`github.com/grafana/jsonnet-libs/helm-util`](https://github.com/grafana/jsonnet-libs/tree/master/helm-util)
-library.  
+library.  Install it with:
+```bash
+jb install github.com/grafana/jsonnet-libs/helm-util
+```
+
 The following example shows how to extract the individual resources of the
 [`grafana`](https://hub.helm.sh/charts/grafana/grafana) Helm Chart: