[Python][Docs] Tensor canonical type extension example

[AlenkaF]

Describe the enhancement requested

We propose to add a canonical extension type for tensors and this issue is adding an example of the implementation and the usage of it in PyArrow. Mainly to be added to the ML together with the proposed canonical type specification.

Component(s)

Python

[AlenkaF]

Closing this issue as the vote for the proposed canonical type specification for tensors has passed and the implementation of the extension type will be added into C++ with #15483. The examples from the linked PR can be reused once the Python binding to the C++ implementation will be added.