Update "Adding an op" doc

[azecevicTT]

Matmul that was taken as example of an op to add was updated since doc was first written. The doc should be updated accordingly together with some pieces of information that might have changed in the meantime. Also it should include some references to decomposition/canonicalization that were added in the meantime, since this is a doc that most people new to the tt-mlir will read.

https://docs.tenstorrent.com/tt-mlir/adding-an-op.html