Update Documentations

[negin513]

@dcherian and I brainstormed on the topics that should be included in the docs as following:

1. Basic Concepts :

• How to convert numpy and cupy.
• .mean , .std and basic operations.
• Creating Arrays
• Checking for cupy arrays
• Accessing the underlying array
• Moving data between CPU and GPU (or host and device)

2. Fundamental Xarray + Cupy:

• Most Xarray operations preserve array type
• Alignment
• Broadcasting
• Basic Arithmetic
• Numpy universal functions

3. High-level Xarray functions

• Groupby works
• Rolling windows do not work
• Weighted operations work

Make the docs as notebooks.

[jacobtomlinson]

I'm not sure I understand this issue. What are you asking here?