Restructure user guide with subsections (pvlib#2310)

* initial creation

* return glossary to user guide

* revert learning page

* add section label to index.rst

* delete learning

* Update index.rst

* update extras maxdepth in index.rst

* user guide label test

* remove user guide label test (passed)

* remove learning from index

* change subsection title -> modeling topcis

Co-Authored-By: Kevin Anderson <57452607+kandersolar@users.noreply.github.com>

* Update v0.11.3.rst

---------

Co-authored-by: Kevin Anderson <57452607+kandersolar@users.noreply.github.com>

Author: 2 people

docs/sphinx/source/user_guide/index.rst

@@ -1,20 +1,34 @@
+.. _user_guide:
+
 ==========
 User Guide
 ==========
 
+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
-   clearsky
    bifacial
+   clearsky
    weather_data
    singlediode
+
+.. toctree::
+   :caption: Extras
+   :maxdepth: 2
+
    nomenclature
    faq

docs/sphinx/source/whatsnew/v0.11.3.rst

@@ -14,6 +14,7 @@ Enhancements
 
 Documentation
 ~~~~~~~~~~~~~
+* Restructure the user guide with subsections (:issue:`2302`, :pull:`2310`)
 
 
 Testing