docs: Remove headers made redundant by ToC captions

docs/index.rst

@@ -74,9 +74,6 @@ If you want to stay up to date on Mopidy developments, you can follow `@mopidy
 announcements related to Mopidy and Mopidy extensions.
 
 
-Usage
-=====
-
 .. toctree::
     :caption: Usage
     :maxdepth: 2
@@ -90,9 +87,6 @@ Usage
 
 .. _ext:
 
-Extensions
-==========
-
 .. toctree::
     :caption: Extensions
     :maxdepth: 2
@@ -110,9 +104,6 @@ Extensions
     ext/web
 
 
-Clients
-=======
-
 .. toctree::
     :caption: Clients
     :maxdepth: 2
@@ -123,9 +114,6 @@ Clients
     clients/upnp
 
 
-About
-=====
-
 .. toctree::
     :caption: About
     :maxdepth: 1
@@ -136,9 +124,6 @@ About
     versioning
 
 
-Development
-===========
-
 .. toctree::
     :caption: Development
     :maxdepth: 2
@@ -150,9 +135,6 @@ Development
     extensiondev
 
 
-Reference
-=========
-
 .. toctree::
     :caption: Reference
     :maxdepth: 2