Deprecate the doc url at /en/master/ in favor of /en/latest/

docs/conf.py

@@ -78,6 +78,8 @@ html_theme_options = {
     "show_related": False,
 }
 
+html_extra_path = ["robots.txt"]
+
 # Custom sidebar templates, must be a dictionary that maps document names
 # to template names.
 #

docs/robots.txt

@@ -0,0 +1,2 @@
+User-agent: *
+Disallow: /en/master/