Add PDF documentation build

This commit adds PDF documentation build target 'pdf-docs' that will
build PDF versions of our docs. As per the Train community goal:

 https://governance.openstack.org/tc/goals/selected/train/pdf-doc-generation.html

Change-Id: Iecb0fe5b957af7dae66bea04dfbd9c2fb4f74a99
Story: #2006070
Task: #35456
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 8f95858..6694022 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -11,9 +11,8 @@
    and how to go beyond this setup. Both should be a set of quick
    links to other documents to let people explore from there.
 
-==========
- DevStack
-==========
+DevStack
+========
 
 .. image:: assets/images/logo-blue.png
 
@@ -32,7 +31,7 @@
    are dedicated to this purpose.
 
 Quick Start
-===========
++++++++++++
 
 Install Linux
 -------------
@@ -153,7 +152,7 @@
 <hacking>`.
 
 Contents
---------
+++++++++
 
 .. toctree::
    :glob: