Add jump.next/prev to documentation

[gKits]

I feel like this would be a nice addition to be able to press ]t and [t to jump to the next or previous test case inside the current buffer.

So it would be cool if the methods neotest.previous_case and neotest.next_case would be provided. What are your opinions about this topic?

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Edit:

I just found out that this functionality already exists but was not documented inside the README. So it would be nice if someone could add a section in the documentation.