[docs] move 'settings.yml' into admin's 'Engines & Settings'

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

docs/admin/engines/index.rst

@@ -12,6 +12,7 @@ Engines & Settings
 .. toctree::
    :maxdepth: 1
 
+   settings
    configured_engines
    private-engines
    recoll

docs/admin/index.rst

@@ -13,7 +13,6 @@ Administrator documentation
    installation-apache
    installation-docker
    update-searx
-   settings
    engines/index
    api
    architecture