Docs: Add MD for only-builtins rule

src/ansiblelint/rules/only_builtins.md

@@ -0,0 +1,36 @@
+# only-builtins
+
+This rule checks that playbooks use actions from the `ansible.builtin` collection only.
+
+This is an opt-in rule.
+You must enable it in your Ansible-lint configuration as follows:
+
+```yaml
+enable_list:
+  - only-builtins
+```
+
+## Problematic Code
+
+```yaml
+---
+- name: Example playbook
+  hosts: all
+  tasks:
+    - name: Deploy a Helm chart for Prometheus
+      kubernetes.core.helm: # <- Uses a non-builtin collection.
+        name: test
+        chart_ref: stable/prometheus
+        release_namespace: monitoring
+        create_namespace: true
+```
+
+## Correct Code
+
+```yaml
+- name: Example playbook
+  hosts: localhost
+  tasks:
+    - name: Run a shell command
+      ansible.builtin.shell: echo This playbook uses actions from the builtin collection only.
+```