Add documentation about automatic client updater

doc/updater.md

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+# Updater
+
+## Description / Use Cases
+
+The updater service allows clients to be updated automatically. Changes in the Ansible playbook or inventory are applied automatically to all configured clients.
+
+## Requirements
+
+* The Ansible playbook must be available via a **public Git repository**.
+* Hosts in the inventory must be specified by their **hostname**, not by their IP address.
+
+## Activation / Default
+
+To install the updater service on clients, set the variable `misc_updater_repository` to a valid Git repository URL. If not set (default: **false**), the service will not be installed.
+
+To enable the updater service on clients, set the variable `misc_updater_autostart` to **true**.
+
+## Configuration / Variables
+
+* **misc_updater_inventory**  
+  Name of the Ansible inventory file.  
+  Type: **String**
+
+* **misc_updater_inventory_password**  
+  Password for the inventory file if it is protected or encrypted via Ansible Vault.  
+  Type: **String**
+
+* **misc_updater_repository**  
+  URL of the public Git repository.  
+  Type: **String**
+
+* **misc_updater_branch**  
+  Name of the Git branch.  
+  Type: **String**
+
+* **misc_updater_autostart**  
+  Flag to enable the updater service. The updater starts 5 minutes after boot.  
+  Type: **Boolean**  
+  Values:
+  * `false` — default
+  * `true`
+
+## Example
+
+All computers receive the updater service.
+Automatic updates are installed on all computers except those in the teacherlaptops group.
+
+```
+inventory.yml
+all:
+  vars:
+    misc_updater_inventory: inventory-myschool.yml
+    misc_updater_inventory_password: top-secret
+    misc_updater_repository: https://git.example.com/it-team/lmn-client.git
+    misc_updater_branch: myschool
+    misc_updater_autostart: true
+
+teacherlaptops:
+  vars:
+    misc_updater_autostart: false
+```