deprecated-module: add documentation

src/ansiblelint/rules/deprecated_module.md

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+# deprecated-module
+
+This rule identifies deprecated modules in playbooks.
+You should avoid using deprecated modules because they are not maintained, which can pose a security risk.
+Additionally when a module is deprecated it is available temporarily with a plan for future removal.
+
+Refer to the [Ansible module index](https://docs.ansible.com/ansible/latest/collections/index_module.html) for information about replacements and removal dates for deprecated modules.
+
+## Problematic Code
+
+```yaml
+---
+- name: Example playbook
+  hosts: localhost
+  tasks:
+    - name: Configure VLAN ID
+      ansible.netcommon.net_vlan: # <-- This module is deprecated.
+        vlan_id: 20
+```
+
+## Correct Code
+
+```yaml
+---
+- name: Example playbook
+  hosts: localhost
+  tasks:
+    - name: Configure VLAN ID
+      dellemc.enterprise_sonic.sonic_vlans: # <-- Use a platform specific module.
+        config:
+          - vlan_id: 20
+```