add captions back in

Author: RDaxini

docs/sphinx/source/user_guide/index.rst

@@ -16,6 +16,7 @@ This user guide is an overview and explains some of the key features of pvlib.
 
 .. toctree::
    :maxdepth: 2
+   :caption: Modeling topics
 
    modeling_topics/pvsystem
    modeling_topics/modelchain
@@ -28,6 +29,7 @@ This user guide is an overview and explains some of the key features of pvlib.
 
 .. toctree::
    :maxdepth: 2
+   :caption: Extras
 
    extras/nomenclature
    extras/faq