On wikipedia there are some results on Itô's lemma applied to non-continuous semimartingales (sometimes called Itô-Doeblin lemma)
I have looked up the books by Malliaris, Oksendal and Doob, but they only mention the case of Poisson jumps, not the more general case.
Where can I find a reference and a demonstration of Itô's lemma for non-continuous semimartingales?
As saz pointed out, there are several books on jump processes which give demonstrations and examples on semimartingales. Some of these books are: