docs: Fix RTD build configuration.

ReadTheDocs needs the root directory in its search path so it finds
topindex.html.

docs/conf.py

@@ -114,6 +114,8 @@ if not on_rtd:  # only import and set the theme if we're building docs locally
     except:
         html_theme = 'default'
         html_theme_path = ['.']
+else:
+    html_theme_path = ['.']
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the