in /_common/, added democode.rst, demonstrating markup and text for an admonition to customize sample code. If I had access to the design, I'd make a note like this float alongside every block of sample code, shimmering or breathing in some way so as to look alive and attract attention. I don't have a way to do that, tho, so I'm just adding democode.rst in case it inspires someone to figure out how to tie this kind of reminder to all the samples.