Add xarray index for docs

bob/extension/utils.py

@@ -534,6 +534,7 @@ def link_documentation(additional_packages = ['python', 'numpy'], requirements_f
   _add_index('distributed', 'https://distributed.dask.org/en/latest/')
   _add_click_index()
   _add_index('torch', 'https://pytorch.org/docs/')
+  _add_index('xarray', 'https://xarray.pydata.org/en/stable/')
 
 
   # get the server for the other packages