[doc] add some documentation about the limiter plugin (and redis)

Requested-by: https://github.com/searxng/searxng/discussions/993#discussioncomment-2396914
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/admin/filtron.rst

@@ -5,17 +5,19 @@
 How to protect an instance
 ==========================
 
+.. tip::
+
+   To protect your instance a installation of filtron (as described here) is no
+   longer needed, alternatively activate the :ref:`limiter plugin` in your
+   ``settings.yml``. Note that the :ref:`limiter plugin` requires a :ref:`Redis
+   <settings redis>` database.
+
+
 .. sidebar:: further reading
 
    - :ref:`filtron.sh`
    - :ref:`nginx searxng site`
 
-
-.. contents:: Contents
-   :depth: 2
-   :local:
-   :backlinks: entry
-
 .. _filtron: https://github.com/searxng/filtron
 
 SearXNG depends on external search services.  To avoid the abuse of these services