Add examples for using hillshade layers in iOS docs

[mollymerp]

Blocked upstream until #11025 #10991 are closed

[1ec5]

#11036 adds a very handwavy example to go in the MGLHillshadeStyleLayer class reference for iOS/macOS, based on this GL JS example. We could also add a more full-fledged example to the ios-sdk-examples project (which winds up on mapbox.com).

/cc @jmkiley

[tobrun]

I ticketed out mapbox/mapbox-android-demo#609 to add an example to our Mapbox Demo application. Once implemented that example should be added to the website. Since we have a ticket tracking the Android example, I'm going to remove the android label.

cc @langsmith

[1ec5]

I think iOS is also covered between #11036 and mapbox/ios-sdk-examples#144. Closing.