Add xarray to intersphinx_mapping

[rocco8773]

Since we've started utilizing xarray's we need to add its object mappings to the intersphinx_mapping config variable in /docs/conf.py. This will allow intersphinx to create the appropriate links to associated documentation when we write something like `xarray.DataArray` in the documentation.

[namurphy]

Adding the "upstream fix required" since (my current understanding is that) xarray needs to make itself compatible with intersphinx before we can do this.

Related to: pydata/xarray#4279