doc: Enable parallel build

"-j auto" enable parallel documentation build with multiple processes
and the number of processors is detected automatically.
This speeds up local documentation build a lot.

Change-Id: I7fab3a37a4a15b886d3456eebad86115a1a63f11

tox.ini

@@ -126,7 +126,7 @@ deps =
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands =
-  sphinx-build -W -b html doc/source doc/build/html
+  sphinx-build -j auto -W -b html doc/source doc/build/html
   doc8 doc/source
 
 [testenv:pdf-docs]
@@ -136,7 +136,7 @@ deps = {[testenv:docs]deps}
 whitelist_externals =
   make
 commands =
-  sphinx-build -W -b latex doc/source doc/build/pdf
+  sphinx-build -j auto -W -b latex doc/source doc/build/pdf
   make -C doc/build/pdf
 
 [testenv:releasenotes]
@@ -217,4 +217,4 @@ markers =
 python_files =
   test_*.py
   *_test.py
-  tests.py
+  tests.py