Add documentation for CustomScript addon feature

Co-authored-by: humitos <244656+humitos@users.noreply.github.com>

Author: Copilot

docs/user/addons.rst

@@ -388,6 +388,61 @@ Here's a complete example showing how to create a version selector using the Add
       }
     );
 
+Custom script
+-------------
+
+The Custom Script addon allows you to inject a custom JavaScript file into your documentation at serve time.
+This enables you to modify or enhance frozen documentation without rebuilding it.
+
+.. note::
+
+   The Custom Script addon is not currently exposed in the user interface.
+   If you would like to use this feature,
+   please :doc:`contact support </support>`.
+
+Using custom scripts
+~~~~~~~~~~~~~~~~~~~~
+
+This addon allows specifying a URL to a JavaScript file that will be injected into all pages of your documentation.
+The script can be hosted on Read the Docs itself (as a relative URL) or on an external server (as an absolute URL).
+
+Common use cases for custom scripts include:
+
+* Fixing bugs or adding features to frozen documentation
+* Injecting analytics or tracking code
+* Customizing the appearance or behavior of specific documentation versions
+
+Accessing Addons data from your script
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Custom scripts are loaded after the ``readthedocs-addons-data-ready`` event has been fired.
+To access the Addons data from your custom script,
+check the ``window.ReadTheDocsEventData`` object first,
+then subscribe to the event for future updates (for example, when the URL changes in a single-page application):
+
+.. code-block:: javascript
+
+    function handleReadTheDocsData(data) {
+      // Access the Addons data here
+      console.log("Project slug:", data.projects.current.slug);
+
+      // You can filter by version to apply changes selectively
+      if (data.versions.current.slug === "v3.0") {
+        // Do something specific for version v3.0
+      }
+    }
+
+    // The event has already fired when custom scripts load.
+    // Check for window.ReadTheDocsEventData first.
+    if (window.ReadTheDocsEventData !== undefined) {
+      handleReadTheDocsData(window.ReadTheDocsEventData.data());
+    }
+
+    // Subscribe to the event for future data updates
+    document.addEventListener("readthedocs-addons-data-ready", function (event) {
+      handleReadTheDocsData(event.detail.data());
+    });
+
 Diving deeper
 -------------