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/tox.ini b/tox.ini
index d81107f..26baa2a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -43,6 +43,14 @@
 commands =
   sphinx-build -W -b html -d doc/build/doctrees doc/source doc/build/html
 
+[testenv:pdf-docs]
+basepython = python3
+deps = {[testenv:docs]deps}
+whitelist_externals =
+   make
+commands =
+   sphinx-build -W -b latex doc/source doc/build/pdf
+   make -C doc/build/pdf
 
 [testenv:venv]
 basepython = python3