Add docs for defining "events" on viewables
Based on [this discord discussion]( https://discord.com/channels/371759389889003530/979844494205812806/1249385031298711724)

It appears this particular feature is not yet well documented and could use an example in the book.

Could be written as an additional section in the [Custom Widget](https://can-lehmann.github.io/owlkettle/book/internals/custom_widgets.html) docs.
