Add tutorial on creating a variational ansatz

[kevinsung]

The tutorial should demonstrate creating a class for representing a variational ansatz and simulating it. The UCJSpinBalanced class can be used as a model. In particular, the _apply_unitary_ method should be defined on the class. Some examples of simulation can be found in https://qiskit-community.github.io/ffsim/how-to-guides/simulate-lucj.html. We can either recreate a simpler version of the UCJ ansatz, or we can create a new ansatz that is not in ffsim (perhaps we can search the literature for one).

[q-inho]

I'm interested in exploring and creating this tutorial. Do you already have an assignee for this task?
If not, I'm would like to share candidate ansätze for your review after an initial exploration.

[0sophy1]

Thank you, Kevin. This is a great initiative. @q-inho I think this issue will be a very good Qiskit Advocate Mentoring project. I think we should talk more on Slack.