Update the stochastic parameter-shift demo to link to/reference recent works and demos

[josh146]

The stochastic parameter-shift demo is somewhat outdated in terms of the content, and doesn't reflect new understanding of where the stochastic shift rule applies. For example:

• it is now known to not apply in general, there remains a restriction on the form of the Hamiltonian of the operator to be differentiated.
• A general parameter-shift rule is known, see https://pennylane.ai/qml/demos/tutorial_general_parshift.html
• In addition, a generalization to the stochastic parameter shift rule is known, which extends the operators it can differentiate, see https://arxiv.org/abs/2107.12390

Note: to address these points, the demo does not need to be substantially re-written or altered. Instead, these points can be addressed simply by moderating a few statements here and there, and adding in relevant links.