Add Drivers to documentation

Re-organize the "internals" section into a "Developer's Guide" with
sub-pages and add the drivers documentation as one of them.

Change-Id: Ibdf0287a7a077dfeaac6c63d1c8beca8d5f3f4be
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 8c12138..784fc4d 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -24,7 +24,7 @@
    launchers
    statsd
    client
-   internals
+   developer
 
 Indices and tables
 ==================