DOC: Add examples to the merge_sort documentation

[seberg]

The merge sort documentation currently describes the parameters and results. It would be good to add a simple example at the end of the current documentation.

Working on this issue

This means finding the merge_sort implementation and modifying the docstring to insert something like:

Examples
--------
>>> ...
result
>>> ...
example

Make sure that the output is exactly as on an interactive terminal, and that the inputs in >>> conform to the style.
(We are currently not testing correctness in CI, but many projects do.)

The examples can include explaining tests if there is a blank line after each example. See https://numpydoc.readthedocs.io/en/latest/format.html#examples for more information.