chore(developer): Deprecate files thumbnail API

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>

developer_manual/app_publishing_maintenance/app_upgrade_guide/index.rst

@@ -11,6 +11,7 @@ These sub pages will cover the most important changes in Nextcloud, as well as s
 .. toctree::
    :maxdepth: 1
 
+   upgrade_to_32.rst
    upgrade_to_31.rst
    upgrade_to_30.rst
    upgrade_to_29.rst

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_32.rst

@@ -0,0 +1,55 @@
+=======================
+Upgrade to Nextcloud 32
+=======================
+
+General
+-------
+
+- TBD
+
+Front-end changes
+-----------------
+
+Added APIs
+^^^^^^^^^^
+
+- TBD
+
+Changed APIs
+^^^^^^^^^^^^
+
+- TBD
+
+Deprecated APIs
+^^^^^^^^^^^^^^^
+
+- TBD
+
+Removed APIs
+^^^^^^^^^^^^
+
+- TBD
+
+Back-end changes
+----------------
+
+Added APIs
+^^^^^^^^^^
+
+- TBD
+
+Changed APIs
+^^^^^^^^^^^^
+
+- TBD
+
+Deprecated APIs
+^^^^^^^^^^^^^^^
+
+- The files API endpoint ``/apps/files/api/v1/thumbnail/`` for generating previews is deprecated.
+  Instead use the preview endpoint provided by Nextcloud core (``/core/preview``).
+
+Removed APIs
+^^^^^^^^^^^^
+
+- TBD