feat(xml): add spinbox support (#8957)

Co-authored-by: Liam Howatt <30486941+liamHowatt@users.noreply.github.com>

docs/src/details/widgets/spinbox.rst

@@ -78,6 +78,22 @@ Events
     Learn more about :ref:`events`.
 
 
+Data binding
+------------
+
+To get familiar with observers, subjects, and data bindings in general visit the
+:ref:`Observer <observer_how_to_use>` page.
+
+This method of subscribing to an integer Subject affects a Subject Widget's integer
+value directly.  Note that this is a two-way binding (Subject <===> Widget), so an end
+user's direct interaction with the Spinbox Widget updates the Subject's value and vice
+versa.
+
+It supports integer subjects.
+
+
+- :cpp:expr:`lv_spinbox_bind_value(spinbox, &subject)`
+
 
 .. _lv_spinbox_keys: