Update documentation to remove pennylane.optimize

[CatalinaAlbornoz]

Feature details

Remove the mention to pennylane.optimize in the documentation since it no longer exists. Eg remove it from here.

Implementation

No response

How important would you say this feature is?

1: Not important. Would be nice to have.

Additional information

No response

[albi3ro]

Should we instead add module documentation for the optimize module?

[trbromley]

I'm not sure why we decided not to add pennylane.optimize module documentation in the first place.

But looking at the description here, I'm not sure what more we'd add. So maybe it's enough to just edit the line on that page: "For more details on the NumPy optimizers, check out the tutorials, as well as the pennylane.optimize documentation".

[josh146]

We opted to remove pennylane.optimize since everything in it lives top level, and it was causing confusion to Sphinx and readers to have the same function live in two places.

The same reason why we don't have module docs for qml.ops.

Rather than add it back in, I would take this as a datapoint we should make the optimization quickstart clearer and easier to find