Ahodges/documentation/doc253

@frrrances @sarina @srpearce @mhoeber 
Please take a look, these are the doc updates for the new button in LMS that navigate back to Studio, and the restyling of the Submission History and Staff Debug links as buttons. (feature already merged)
NB: there are a number of screen shots in the doc that technically should now be retaken. I believe that task has a lower priority than getting the new feature documented, and it will take a significant amount of time to do the setup, so I have identified the images and opened a separate DOC issue to get them retaken in a subsequent sprint.