Integrate Leaflet docs on page, add 1.6.0 docs

This integrates Leaflet docs inline with mapbox.js docs.

* How do you pull a specific version of Leaflet docs? doesn't seem like we can use the npm package or git tags. /cc @mourner 
* This makes the page extremely long, too long in my opinion for actual use. I think this needs more structure like... collapsible sections, or mapbox.js & leaflet on different pages, or something else? Thoughts @tristen ?
* Leaflet's `reference.html` is not one thing - it's a TOC, an intro, and Javascript code. I've removed this manually for the time being, and could write some javascript `.replace()` to do the same thing, but this seems icky and hacky and I would like not to. /cc @mourner