Update whatsnew and set rst default role

Updating the whatsnew doc from the PR files is the easy bit (but please remember to write your whatsnew entries as bullet points or subheadings - it's a bit fiddly to sort them out without those delimiters).

I've also set the rst default role, which makes single backticks work like double backticks (i.e. like inline code in markdown), since people seem to be assuming that works anyway. The only argument I can see against doing this is that it makes our rst slightly less portable to other environments, but I think that's OK.

    `code` wlll now render the same as ``code``