GH-33947: [Python][Docs] Tensor canonical type extension example

[AlenkaF]

Rationale for this change

There have been quite a lot of discussions connected to the tensor support in Arrow Tables/RecorBatches (#33924). This PR is a Python implementation example for the specification proposal to add tensors as a canonical type extensions which will be sent to the Mailing list for discussion and vote.

What changes are included in this PR?

• Implementation of the fixed size tensor canonical extension type in PyArrow plus tests (as an example in test_extension_type.py)
• Example of the implementation and example of the usage in pyarrow.Table (added to extending_types.rst)

Both are only illustrative, do not merge!

Are these changes tested?

• Tests for the implementation are added to test_extension_type.py

Are there any user-facing changes?

No.

• Closes: [Python][Docs] Tensor canonical type extension example #33947

cc @rok @jorisvandenbossche

[github-actions]

• Closes: [Python][Docs] Tensor canonical type extension example #33947

[AlenkaF]

Closing in favour of #34883 and #34957