Make guide headings have anchors

I'm hoping to do another round on the guides soon. First step was to make it so you could link to the sections. 

I moved the guides into `_guides`, now they're rendered into `_templates/guide.html` with `bin/generate_guides`. 

Advantages: 

- Can insert `<a>`s into the headers
- Github-style fenced code blocks instead of `{% highlight %}`
- Autolinking for class names, like the API docs

Hoping to do a table of contents and sub-guides soon...