create user guide subection

Author: RDaxini

docs/sphinx/source/user_guide/index.rst

@@ -9,24 +9,29 @@ This user guide is an overview and explains some of the key features of pvlib.
 .. toctree::
    :caption: Getting started
    :maxdepth: 2
-   
+
    package_overview
    installation
    introtutorial
-   
+
 .. toctree::
    :caption: Modeling topics
    :maxdepth: 2
-   
+
    pvsystem
    modelchain
    timetimezones
    bifacial
    clearsky
    weather_data
-   model_comparison
    singlediode
 
+.. toctree::
+   :caption: Model comparisons
+   :maxdepth: 2
+
+   model_comparison
+
 .. toctree::
    :caption: Extras
    :maxdepth: 2