Fix broken links to static html doc files (issue2550840)

It seems as though these links have been broken every since sphinx has
been used to generate the documentation. Version 1.2 of sphinx
introduced the ability to include extra static files, so we are making
use of this facility to fix the links to static html files.

Author: jerrykan

doc/conf.py

@@ -109,6 +109,11 @@
 # given in html_static_path.
 html_style = 'default.css'
 
+# Add any extra paths that contain custom files (such as robots.txt or
+# .htaccess) here, relative to this directory. These files are copied
+# directly to the root of the documentation.
+html_extra_path = ['html_extra']
+
 # The name for this set of Sphinx documents.  If None, it defaults to
 # "<project> v<release> documentation".
 #html_title = None