revert to @allcaps's original approach to including robots.txt

docs/conf.py

@@ -163,7 +163,7 @@ html_static_path = ['_static']
 # 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']
+html_extra_path = ['robots.txt']
 
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.