Online documentation

Moved from https://github.com/mapbox/mapbox-android-sdk/pull/402

This commit add's a `docs` directory to the main repo and creates online developer documentation for Mapbox Android SDK. To generate the API docs I've added instructions here: https://github.com/mapbox/mapbox-android-sdk/tree/mb-pages/docs

- [x] API Interlinks are broken: http://localhost:4000/api/0100/01/01/maplistener.html/ links to http://localhost:4000/api/ZoomEvent

### Post merge
- [ ] Set up [publisher](https://github.com/mapbox/publisher/) to update changes.
- [ ] Add `mapbox-android-sdk` to [sync-templates](github.com/mapbox/sync-templates).
- [ ] Merge https://github.com/mapbox/www.mapbox.com/pull/4149