API docs for xtensor module

[ricardoV94]

Description

We should list all the public-facing functions / methods of the xtensor module.

We should also have a page on notable differences between xarray and xtensor variables:

• No coordinate-based operations
• outputs and inputs are still numpy arrays, only treated as xarray DataArrays for the purpose of constructing the computational graph
• Unique operations: unstack, linalg (relationship to xarray-einstats API...), random variables