Support standalone output

[RalfJung]

Currently it seems like the only supported output format is mdbook or raw dot files. That's not a great match when just rendering one skill tree. Consider https://rust-lang.github.io/const-eval/ -- there are several usability problems:

• The ToC on the left is almost empty, so it is not even clear that there is a ToC there. For the first 5 minutes I thought there was a random stripe of whitespace that the page did not use for some reason. It is also not easy to see that you can hide this menu ("Toggle table of contents" icon is hard to recognizable rust-lang/mdBook#1296).
• The horizontal scrollbar is only visible after scrolling all the way down (Bad handling of horizontal scroll bars rust-lang/mdBook#1297).

For our usecase in const-eval, it would be much better if we could just render to a single HTML page.

[nikomatsakis]

I think we still support the old mode, though I suspect that the web page should maybe just have more content...

[nikomatsakis]

but yeah, maybe I was too quick to remove the old suppor

[RalfJung]

So how'd the old mode be used now?