Parallelize docs build

This makes sphinx-build use the number of cores available to make the
docs build parallel. On my 16 core system, this cut the time by ~50%. We
don't have as many cores available in the gate, but it should provide a
slight improvement in overall job time.

Change-Id: I7ad4d87a3dba10475a0fcf797a9aa2d4b1ad0eb5
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

doc/source/_exts/deliverables.py

@@ -523,3 +523,7 @@ def setup(app):
     app.add_directive('serieshighlights', HighlightsDirective)
     app.connect('build-finished', build_finished)
     _generate_team_pages()
+    return {
+        'parallel_read_safe': True,
+        'parallel_write_safe': True,
+    }

doc/source/_exts/ics.py

@@ -230,3 +230,7 @@ def setup(app):
     app.add_directive('ics', ICS)
     app.connect('doctree-resolved', doctree_resolved)
     app.connect('build-finished', build_finished)
+    return {
+        'parallel_read_safe': True,
+        'parallel_write_safe': True,
+    }

tox.ini

@@ -75,7 +75,7 @@ deps =
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands =
-  sphinx-build -v -a -E -W -d doc/build/doctrees -b html doc/source doc/build/html
+  sphinx-build -v -a -E -W -j auto -d doc/build/doctrees -b html doc/source doc/build/html
   whereto {toxinidir}/doc/build/html/.htaccess {toxinidir}/doc/build/redirect-tests.txt
 
 [flake8]