refer to subsecton index in main index

Author: RDaxini

docs/sphinx/source/user_guide/index.rst

@@ -10,25 +10,16 @@ This user guide is an overview and explains some of the key features of pvlib.
    :caption: Getting started
    :maxdepth: 2
 
-   getting_started/package_overview
-   getting_started/installation
-   getting_started/introtutorial
+   getting_started/index
 
 .. toctree::
    :maxdepth: 2
    :caption: Modeling topics
 
-   modeling_topics/pvsystem
-   modeling_topics/modelchain
-   modeling_topics/timetimezones
-   modeling_topics/bifacial
-   modeling_topics/clearsky
-   modeling_topics/weather_data
-   modeling_topics/singlediode
+    modeling_topics/index
 
 .. toctree::
    :maxdepth: 2
    :caption: Extras
 
-   extras/nomenclature
-   extras/faq
+   extras/index