Refactor rustdoc main.js to not use jQuery

[clarfonthey]

I took a look at the code and it appears that almost all of the stuff that is implemented in jQuery could be implemented in raw JS with a few extra shims, such that it'd work on virtually all modern browsers. I'd be willing to make these changes but I figured I'd open up an issue to see what people think.

jQuery is kind of big and if a user generates docs for a lot of different crates, it tends to add up.

[clarfonthey]

Additional note: I've started some of this but if anyone at any point would like to take over, let me know.