Documentation build drops a file in the working directory

[jakelishman]

Environment

• Qiskit Terra version: main @ 686ba34
• Python version: any
• Operating system: any

What is happening?

Running the documentation build (such as by tox -e docs) causes a file new_hist.png to be dropped in the working directory. This came from #8569, which added a jupyter-execute block to the qiskit.visualization documentation that leads a savefig command.

How can we reproduce the issue?

Run the documentation build.

What should happen?

No files outside the documentation build directory should be created.

Any suggestions?

The offending .. jupter-execute block can just be replaced by a .. code-block one, since it doesn't actually output anything visually, or execute code in the IPython kernel that needs persisting to later cells.