document new functions. link to new example from audio page, index, and ...

...index-offline.

New functions for AudioManager:

 - getTime(sound_name)
 - setTime(sound_name, time)
 - isPaused(sound_name)
 - isPlaying(sound_name)
 - getDuration(sound_name)

Goes with:

 - https://github.com/gameclosure/timestep-priv/pull/74
 - https://github.com/gameclosure/gcapi-priv/pull/21
 - https://github.com/gameclosure/native-core-priv/pull/16
 - https://github.com/gameclosure/native-android-priv/pull/41
 - https://github.com/gameclosure/native-ios-priv/pull/35
 - https://github.com/gameclosure/examples/pull/41