Provide example UI for content authors to use to allow rendering customisation?

[matatk]

Similarly to the idea of doing a WebExtension for end-users (#54) and in support of providing UI on all examples around the site (changes to the gallery coming in #64), a bit of HTML and JS could be created to help content authors include UI in their sites to customise AudioChart's rendering. This would be kept simple, but might help users understand the chart better.

I imagine this would be some sort of interactive pop-up that can be triggered from a button adjacent to the visual chart and the existing "play/pause" button.

Not currently sure how it might relate to selective muting/ducking of series.