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 ?