Document the Dag Versions comparison tab in the UI overview (#76387)

The UI gained a Versions tab on the Dag page that compares two stored
versions of a Dag, but the UI overview page did not mention it. Add a
section next to the other Dag tabs describing how to pick versions, what
the change table shows, who can see values, and the Max changes limit,
with dark and light screenshots.

airflow-core/docs/ui.rst

@@ -334,6 +334,38 @@ The **Details** tab provides configuration details and metadata for the Dag, inc
         .. image:: img/ui-light/dag_overview_details.png
            :alt: Dag Details Tab (Light Mode)
 
+Versions Tab
+''''''''''''
+
+.. versionadded:: 3.4.0
+
+The **Versions** tab compares two versions of the Dag. Pick a version in **Compare from** and another in
+**Compare to**, and the tab lists every change between them: the path of what changed, whether it was added,
+changed or removed, its category (for example task, schedule, param or asset) and its impact. Changes that
+affect execution, such as a task's operator or retries, are marked separately from metadata-only changes like
+a description or tag.
+
+The **Before** and **After** columns, and the names in each path, are shown only to users who can read the
+Dag's code. Other users still see what kind of change was made, with identifying names in the path masked and
+the values marked as hidden. A large comparison is cut at 500 changes by
+default; set **Max changes** (up to 5000) to see more. The selected versions are kept in the URL, so a
+comparison can be shared as a link.
+
+.. tab-set::
+    :sync-group: ui-theme
+
+    .. tab-item:: Dark
+        :sync: dark
+
+        .. image:: img/ui-dark/dag_overview_versions.png
+           :alt: Dag Versions Tab (Dark Mode)
+
+    .. tab-item:: Light
+        :sync: light
+
+        .. image:: img/ui-light/dag_overview_versions.png
+           :alt: Dag Versions Tab (Light Mode)
+
 .. _ui-dag-runs:
 
 Dag Run View