Moved content out of math wiki

As it says on the tin, I moved almost all of the content other than the quick start for math lib users out of the math wiki and into the stan wiki.

The one page I couldn’t find a home for is the detailed description:

The stan-dev/stan wikis could really use a serious pass to reduce redundancy. Over time, a lot of people (including me!) contributed redundant descriptions. I’ve consolidated them all and gotten rid of some straight-up redundancy, but the whole set of doc could (and should!) be greatly streamlined.

I’ll probably get to this myself over time—it’s a great constructive procrastination project. But feel free to jump in and help edit.