/docs/reference/about-the-cue-playground: Colophon for the playground

[jpluscplusm]

In resolving #158 we disabled the /about-play link from the playground's nav menu. If we prioritise re-enabling that link, it should link to somewhere within our existing structure - /docs/reference/about-the-cue-playground feels appropriate.

I believe that this would be suitable for a colophon (or similar) about the playground, and not a guide to using the playground.

If we choose to write a more comprehensive guide to using the playground, it should probably be linked more prominently from /play - perhaps a "HELP!" link in the header, not the nav. Thus this issue tracks the page/content required to satisfy and reinstate only the "About the playground" link in the nav.