Include WG name rather than "spec group"

[wseltzer]

Instead of the outdated spec groups, I'd suggest listing WG under which the spec was produced (not least because that has patent commitment relevance).

[chaals]

There is value in being able to group specs by the concepts they cover. But the categories need to be roughly the same size - the difference between SVG and SVG tiny is irrelevant against "css specs", and there are currently lots of almost pointless categories.

Tags are a helpful approach.