Add Docs.

doc/07_User.md

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+# User
+## Default Key Bindings
+To change the default key bindings, you can add a synfony configuration file in your project. 
+The structure of the file should be like this:
+```yaml
+pimcore_studio_backend:
+  user:
+    default_key_bindings:
+      save:
+        key: 'S'
+        action: save
+        ctrl: true
+```
+
+You can find the predefined key bindings [here](../config/pimcore/user_key_binding.yaml).