[ios] Update and bundle docs screenshot

- Don't use an absolute URL for the screenshot image, include the file instead.
- Add Sketch document for screenshot.

platform/ios/docs/doc-README.md

@@ -2,7 +2,7 @@
 
 The Mapbox iOS SDK is an open-source framework for embedding interactive map views with scalable, customizable vector maps into Cocoa Touch applications on iOS 7.0 and above using Objective-C, Swift, or Interface Builder. It takes stylesheets that conform to the [Mapbox GL Style Specification](https://www.mapbox.com/mapbox-gl-style-spec/), applies them to vector tiles that conform to the [Mapbox Vector Tile Specification](https://www.mapbox.com/developers/vector-tiles/), and renders them using OpenGL.
 
-[![](https://raw.githubusercontent.com/mapbox/mapbox-gl-native/master/platform/ios/screenshot.png)]()
+![Mapbox iOS SDK screenshots](screenshot.png)
 
 For setup information, check out the [Mapbox iOS SDK homepage](https://www.mapbox.com/ios-sdk/). For detailed usage instructions, read “[First steps with the Mapbox iOS SDK](https://www.mapbox.com/help/first-steps-ios-sdk/)” and consult the [online examples](https://www.mapbox.com/ios-sdk/examples/). A [full changelog](https://github.com/mapbox/mapbox-gl-native/blob/master/platform/ios/CHANGELOG.md) is also available.
 

platform/ios/scripts/document.sh

@@ -32,6 +32,8 @@ sed -n -e '/^## /{' -e ':a' -e 'n' -e '/^##/q' -e 'p' -e 'ba' -e '}' platform/io
 rm -rf ${OUTPUT}
 mkdir -p ${OUTPUT}
 
+cp platform/ios/screenshot.png "${OUTPUT}"
+
 jazzy \
     --config platform/ios/jazzy.yml \
     --sdk iphonesimulator \