docs: Add active_item_index example to ui.stack documentation (#1258)

Add active_item_index example of ui.stack to dashboard.md

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: dsmmcken <[email protected]>
Co-authored-by: Don <[email protected]>
Co-authored-by: dsmmcken <[email protected]>

Author: Copilot

plugins/ui/docs/components/dashboard.md

@@ -165,6 +165,21 @@ dash_stack = ui.dashboard(
 )
 ```
 
+### Stack with an active item index
+
+```python
+from deephaven import ui
+
+dash_stack_active = ui.dashboard(
+    ui.stack(
+        ui.panel("A", title="A"),
+        ui.panel("B", title="B"),  # shown as active item
+        ui.panel("C", title="C"),
+        active_item_index=1,
+    )
+)
+```
+
 ### Stack with nested tabs
 
 ```python

plugins/ui/docs/snapshots/caf37df2566b6fe4d58422ae85abd91c.json

@@ -0,0 +1 @@
+{"file":"components/dashboard.md","objects":{"dash_stack_active":{"type":"deephaven.ui.Dashboard","data":{"document":{"props":{"children":{"__dhElemName":"deephaven.ui.components.Stack","props":{"activeItemIndex":1,"children":[{"__dhElemName":"deephaven.ui.components.Panel","props":{"title":"A","direction":"column","alignItems":"start","gap":"size-100","overflow":"auto","padding":"size-100","children":"A"}},{"__dhElemName":"deephaven.ui.components.Panel","props":{"title":"B","direction":"column","alignItems":"start","gap":"size-100","overflow":"auto","padding":"size-100","children":"B"}},{"__dhElemName":"deephaven.ui.components.Panel","props":{"title":"C","direction":"column","alignItems":"start","gap":"size-100","overflow":"auto","padding":"size-100","children":"C"}}]}}},"__dhElemName":"deephaven.ui.components.Dashboard"},"state":"{}"}}}}