Support `deprecated` flag

[bennypowers]

CEM recently got a deprecated flag on most entities. It can either be a boolean or a string deprecation reason. We should support deprecated in the props, slots, events, methods, and css docs viewers.

As an initial suggestion, perhaps deprecated properties could display with a soft red background (set by a css property) with a red border, and the deprecation reason (if present) in italics in a footer of the docs item.