Set noLegacyStyle: true and adjust structure accordingly

See http://www.w3.org/respec/ref.html#nolegacystyle for documentation.

Highlights:
* No auto-generated sections for attributes and methods, so less noise in the ToC and further down.
* The generated sections were sorted alphabetically so often differed from the WebIDL ordering, which is confusing. The hand coded markup is not re-ordered.
This renders the exact same thing as before. The `<dl>` now creates just the WebIDL snippet. The documentation table is what was previously generated by respec. Sadly, we don't have more granular control over what respec does and does not generate. Overall, I'm happy with this patch as it makes the document more readable, less cluttered. The underlying markup is less important.