Improve registry about page

[anjackson]

• Complete the page, with a decent entry for every registry
• More meaningful analysis of 'created' vs 'updated'.
• Note differences like how LoC handles 'look over there for extensions'?
• Can we make the Genre selection show filtered views, that then link on to the registry in question.

[anjackson]

Doing this properly will take time and probably needs more feedback first, so I've simplified things for now and have pushed this back to the next version.