Add example for how to use <MockTemporarySettings> to the docs

Created by: philipp-spiess

Based on feedback from https://github.com/sourcegraph/sourcegraph/pull/32209#discussion_r819778771. I did look at the wiki page before attempting to set up my own mocking so I thought it might be fitting to add an example of it there.

Test plan

Not applicable, this is only a docs change.