Mark sphinx extensions thread safe

This adds the return of some metadata to our sphinx extension setup to
indicate they are thread safe. This is needed to allow consuming
projects to do multithreaded docs builds. In some cases, this can save a
noticeable amount of time in job execution.

Change-Id: I666da30caa825a160e8fd3471615012d0dbfdc22
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

openstackdocstheme/__init__.py

@@ -20,3 +20,7 @@ from .paths import *  # noqa
 def setup(app):
     ext.setup(app)
     page_context.setup(app)
+    return {
+        'parallel_read_safe': True,
+        'parallel_write_safe': True,
+    }

openstackdocstheme/ext.py

@@ -336,4 +336,5 @@ def setup(app):
     )
     return {
         'parallel_read_safe': True,
+        'parallel_write_safe': True,
     }

openstackdocstheme/page_context.py

@@ -98,4 +98,5 @@ def setup(app):
     app.connect('html-page-context', html_page_context)
     return {
         'parallel_read_safe': True,
+        'parallel_write_safe': True,
     }