feat(devmanual): Start 28 upgrade docs

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

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_28.rst
    upgrade_to_27.rst
    upgrade_to_26.rst
    upgrade_to_25.rst

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_28.rst

@@ -0,0 +1,63 @@
+=======================
+Upgrade to Nextcloud 28
+=======================
+
+General
+-------
+
+info.xml
+^^^^^^^^
+
+Make sure your ``appinfo/info.xml`` allows for Nextcloud 28.
+
+.. code-block:: xml
+
+    <dependencies>
+        <nextcloud min-version="25" max-version="28" />
+    </dependencies>
+
+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
+^^^^^^^^^^^^^^^
+
+* tbd
+
+Removed APIs
+^^^^^^^^^^^^
+
+* tbd