Add unstack

[steff456]

This PR adds unstack to the specification, fixes #487.

• Adds the signature inside the manipulation functions
• Adds the reference in the table of contents

[rgommers]

This looks good overall, just some minor comments.

The axis=0 default is right, and the return type should indeed be tuple. I cross-checked with torch.unbind and tf.unstack, and the latter gets the return type wrong: it's annotated as Sequence[Any] and the code can return a list. We've learned that a tuple is better, since this should not be appended to, and a list is actually problematic for at least Numba.

[rgommers]

This LGTM now, thanks @steff456. I'll leave it open for a few days in case there are more comments.

[rgommers]

Merged now, thanks @steff456