Write summary paragraph for synthetic-images tutorial

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trafficstars

One more tutorial that I'd like to see updated is synthetic-images because the Summary section contains a table of contents rather than a summary paragraph. The paragraph is what we'll see in the default listing for tutorials, so it's important (for purposes of the Algolia-based search project) that the summary paragraph is written with that purpose in mind. I could try my hand at writing a summary, but perhaps someone else would be better suited

Left over from #426