Add user-level software installation chapter.

Well, it's a monster, but here it is. I'm pretty excited about this chapter as this is one of the big things I wanted to pass on to current students. It's one of the coolest things I learned while in EOS :grinning:

I wouldn't recommend reading the diff for this one; I would actually build the document and read it like that. Run:
```bash
git fetch
git checkout origin/sean/user-level-software # will say detached head, that's expected
./waf build
# open the built docs
```
I know it's a bit of a burden, but I'd appreciate it if everyone could actually *try* the steps in the chapter in addition to proofreading them. Thanks.