Enclose "How these figures are calculated" section in MUI-theme accordion/details component

As a task spun-off from #15 (closed), we should display the "How these figures are calculated" section in an accordion or details component that uses/conforms with the MUI Theme in order to hide this information until a user decides they want to view it.

In order to do this we should liaise with @pdh45, Jonathan Mitchell and Emmeline Hewstone to get their advice on look-and-feel and design/development of the required component.