Another approach to summaries

Reading one of the survey comments made me think of another approach to 
summaries: put them in an expanding element. I did this in a branch to 
test it out:

https://raw.githack.com/w3c/silver/summary-summary/guidelines/index.html#introduction

If that seems viable I can refine styling etc., just wanted to see how 
it sat compared to other alternatives (summary content squatting in the 
section, summary content moved entirely out of the doc).

Michael