Use zuul-sphinx for configuration layout

This moves the configuration documentation to a hierarchical layout
using the attr directives provided by zuul-sphinx.

Apart from making it look like the zuul documentation, this brings
consistency to things like required flags, default values and typing
info.

There are no content changes but things have moved around somewhat to
accommodate the layout.

Depends-On: https://review.openstack.org/604267

Change-Id: I831dfd8c9458a1f255aa05fa96cfc5c416ed3310

doc/requirements.txt

@@ -2,4 +2,4 @@ sphinx>=1.6.1
 sphinxcontrib-programoutput
 sphinxcontrib-httpdomain
 reno>=2.5.0 # Apache-2.0
-zuul-sphinx
+zuul-sphinx>=0.3.0

doc/source/operation.rst

@@ -222,7 +222,7 @@ is the same.
 Web interface
 -------------
 
-If configured (see :ref:`webapp-conf`), a ``nodepool-launcher``
+If configured (see :attr:`webapp-conf`), a ``nodepool-launcher``
 instance can provide a range of end-points that can provide
 information in text and ``json`` format.  Note if there are multiple
 launchers, all will provide the same information.