Organize the FAQs better

[adehnert]

The FAQ organization could use some love. I think better category split would help --- one contender for a new category is "Security". Also, we now have enough FAQ entries and enough categories that a ToC to the listing page would probably be nice.

(Incidentally, in a the process of doing this, you're probably going to have to rederive how adding categories works and what sortid means. Adding docs of *that* somewhere would be nice.)