[doc] Sans serif font for html documentation.

doc/_static/custom.css

+body{
+    font-family: Ubuntu, sans-serif;
+}

doc/conf.py

@@ -69,6 +69,9 @@ html_theme = 'alabaster'
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ['_static']
+html_css_files = [
+        'custom.css',
+]
 
 # -- Options for LaTeX output ---------------------------------------------