FIX: move sitemap

Author: jklymak

sitemap.xml renamed to _sitemap/sitemap.xml

@@ -6,6 +6,7 @@ User-agent: *
 
 # do not search root directory by default.
 Disallow: /
+Allow: /_sitemap/
 
 # but allow some subdirectories:
 Allow: /stable/
@@ -30,5 +31,6 @@ Allow: /mpl-bench/
 Allow: /mpl-altair/
 Allow: /devdocs/
 
+
 # tell robots this is sitemap
-Sitemap: https://matplotlib.org/sitemap.xml
+Sitemap: https://matplotlib.org/_sitemap/sitemap.xml