Mb pages

Moving this over to https://github.com/mapbox/mapbox-android-sdk/pull/402 as the branch name should really be `mb-pages`

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

### Post merge tasks

- [ ] Move the master default branch to `mb-pages` 
- [ ] Set up publisher to update changes.
- [ ] Add `mapbox-android-sdk` to the [sync-templates](github.com/mapbox/sync-templates) list.
- [ ] Merge https://github.com/mapbox/www.mapbox.com/pull/4149
- [ ] API Interlinks are broken: http://localhost:4000/api/0100/01/01/maplistener.html/ links to http://localhost:4000/api/ZoomEvent