Docs: add toctree, make sidebar nav a bit better

I wanted to have all geocoders listed in the sidebar toctree, because
viewing docs for a specific geocoder is quite a common use-case.
Apparently without introducing level 3 headers that's impossible [1].

sphinx-rtd-theme for some reason cannot render third level of headings
correctly in the sidebar without a `toctree` present in the doc:
they appear at the second level along with the other level two headers.
Hence the need to have `toctree` in the doc. It could've been hidden,
but I figured it might be helpful to have it visible in the doc.
Will see how it goes.

The toctree references the same document, which causes a warning
regarding a circular reference, yet it's the only option I could
find to have the third level of headings be correctly rendered in
the sidebar [2].

[1]: https://stackoverflow.com/a/18002745
[2]: sphinx-doc/sphinx#2103

docs/index.rst

@@ -10,6 +10,12 @@ Welcome to GeoPy's documentation!
 .. automodule:: geopy
    :members: __doc__
 
+.. toctree::
+    :maxdepth: 3
+    :caption: Contents
+
+    index
+
 
 Installation
 ~~~~~~~~~~~~
@@ -30,107 +36,167 @@ Geocoders
 
 .. autofunction:: geopy.geocoders.get_geocoder_for_service
 
+ArcGIS
+------
+
 .. autoclass:: geopy.geocoders.ArcGIS
    :members:
 
    .. automethod:: __init__
 
+AzureMaps
+---------
+
 .. autoclass:: geopy.geocoders.AzureMaps
    :members:
    :inherited-members:
    :show-inheritance:
 
    .. automethod:: __init__
 
+Baidu
+-----
+
 .. autoclass:: geopy.geocoders.Baidu
    :members:
 
    .. automethod:: __init__
 
+Bing
+----
+
 .. autoclass:: geopy.geocoders.Bing
    :members:
 
    .. automethod:: __init__
 
+DataBC
+------
+
 .. autoclass:: geopy.geocoders.DataBC
    :members:
 
    .. automethod:: __init__
 
+GeocodeEarth
+------------
+
 .. autoclass:: geopy.geocoders.GeocodeEarth
    :members:
    :inherited-members:
    :show-inheritance:
 
    .. automethod:: __init__
 
+GeocodeFarm
+-----------
+
 .. autoclass:: geopy.geocoders.GeocodeFarm
    :members:
 
    .. automethod:: __init__
 
+GeoNames
+--------
+
 .. autoclass:: geopy.geocoders.GeoNames
    :members:
 
    .. automethod:: __init__
 
+GoogleV3
+--------
+
 .. autoclass:: geopy.geocoders.GoogleV3
    :members:
 
    .. automethod:: __init__
 
+IGNFrance
+---------
+
 .. autoclass:: geopy.geocoders.IGNFrance
    :members:
 
    .. automethod:: __init__
 
+OpenCage
+--------
+
 .. autoclass:: geopy.geocoders.OpenCage
    :members:
 
    .. automethod:: __init__
 
+OpenMapQuest
+------------
+
 .. autoclass:: geopy.geocoders.OpenMapQuest
    :members:
 
    .. automethod:: __init__
 
+Nominatim
+---------
+
 .. autoclass:: geopy.geocoders.Nominatim
    :members:
 
    .. automethod:: __init__
 
+Pelias
+------
+
 .. autoclass:: geopy.geocoders.Pelias
    :members:
 
    .. automethod:: __init__
 
+Photon
+------
+
 .. autoclass:: geopy.geocoders.Photon
    :members:
 
    .. automethod:: __init__
 
+PickPoint
+---------
+
 .. autoclass:: geopy.geocoders.PickPoint
    :members:
    :inherited-members:
    :show-inheritance:
 
    .. automethod:: __init__
 
+LiveAddress
+-----------
+
 .. autoclass:: geopy.geocoders.LiveAddress
    :members:
 
    .. automethod:: __init__
 
+TomTom
+------
+
 .. autoclass:: geopy.geocoders.TomTom
    :members:
 
    .. automethod:: __init__
 
+What3Words
+----------
+
 .. autoclass:: geopy.geocoders.What3Words
    :members:
 
    .. automethod:: __init__
 
+Yandex
+------
+
 .. autoclass:: geopy.geocoders.Yandex
    :members: