[netinstall] Add example section that is immutable

- The section can't be changed, but is selected (it doesn't
  make sense otherwise)

src/modules/netinstall/netinstall.conf

@@ -83,3 +83,13 @@ groups:
             - zsh
             - zsh-completion
             - zsh-extensions
+    - name: "Kernel"
+      description: "Kernel bits"
+      hidden: false
+      selected: true
+      critical: true
+      immutable: true
+      packages:
+        - kernel
+        - kernel-debugsym
+        - kernel-nvidia