wrap summary in div instead of paragraph

Erik Faye-Lund requested to merge kusma/mesa3d.org:use-div-for-summary into master

It's entirely possible for summaries to contain paragraphs, so let's wrap it in a div instead. And let's add a margin at the bottom of it, to make the spacing consistent.

This should render the same as before, but produce less warnings when validating using Tidy.

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