[doc] add tagesschau API to the debveloper documentation

supplement to the commit e25d1c728

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/dev/engines/online/tagesschau.rst

@@ -0,0 +1,13 @@
+.. _tagesschau engine:
+
+==============
+Tagesschau API
+==============
+
+.. contents:: Contents
+   :depth: 2
+   :local:
+   :backlinks: entry
+
+.. automodule:: searx.engines.tagesschau
+  :members: