Start running our doc builds in nitpicky mode.

This should catch any missing references and other issues.

It's expected to fail currently,
and we should update this branch until things pass properly.

tox.ini

@@ -22,10 +22,10 @@ commands =
     py.test {posargs}
 
 [testenv:docs]
-description = build readthedocs documentation
+description = build readthedocs documentation in nitpicky mode -- warn on errors
 changedir = {toxinidir}/docs
 commands =
-    sphinx-build -b html -d {envtmpdir}/doctrees . {envtmpdir}/html
+    sphinx-build -W -n -b html -d {envtmpdir}/doctrees . {envtmpdir}/html
 
 [testenv:docs-lint]
 description = run linter (rstcheck) to ensure there aren't errors on our docs