Provide migration instructions for the great stylesheet update

[lornajane]

With the changes in #988 people may need to update the stylesheets they are using to keep the PDFs generating.

Make sure that: the manual information for stylesheets is updated (refactoring is definitely welcome; it isn't the easiest topic to get to grips with) and that we specifically add instructions for the upgrade from pre-1.0 systems, including the old stylesheet which is available, and perhaps links to the github history for the old style files or anything else that people might need. Also on how to update existing custom stylesheets; perhaps we should add this to the style conversion script that takes the old rson format and produces yaml.

Figure out the docs site also needs updates to its examples, more examples, or something else (open more issues over there as it becomes clear!)