Add optional content groups example

[ratmice]

I used AI for help with the pdf optional content groups.
It took quite a bit of manual fiddling to both get something that worked with poppler + pdf.js and also didn't drop into manual Content::op() encoding.

I've tried to both clean this up, and trim out any unnecessary bits, but perhaps with my unfamiliarity with ocg some useless garbage may have snuck in. It appears to work without the View/Usage Intents, as those are optional. But I'm not entirely certain how those get used by consumers, so I left them.

I just figured since I went through the trouble myself, perhaps it could help someone else
if there was an example.

[ratmice]

Hmm, suppose I should include a second shape layer to act as a radio button like layer, that would select between the two shape layers. As that is a feature not currently shown...

Should be fixed in c9a2563