[doc] html_unescape: Create html.unescape extension

This new sphinx extension runs html.unescape
(from the Python Standard Library) on source files before they are
rendered, allowing escape sequences like  ' ' for the no-break
space character.

I have also published this extension in my own name under a different
license (the same one Sphinx uses) for others to use:
https://github.com/JJRcop/sphinxcontrib-html_unescape

doc/conf.py

@@ -37,6 +37,7 @@ rst_prolog = """
 
 extensions = [
     'sphinx_rtd_theme',
+    'html_unescape',
 ]
 
 try:
@@ -64,6 +65,14 @@ exclude_patterns = ['_build', 'Thumbs.db', '.DS_Store']
 # Explicitly state master_doc instead of relying on default
 master_doc = 'index'
 
+# Documents to exclude from html_unescape
+# For example ['index', 'credits']
+html_unescape_exclude = []
+
+# If not empty, html_unescape will ignore any documents not in the list
+# For example ['install', 'usage']
+html_unescape_onlyinclude = []
+
 # -- Options for HTML output -------------------------------------------------
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for