Add Example of Converting a MultibandTileLayerRDD[K] to a RasterFrame in the Docs

[jbouffard]

In the Creating RasterFrames section of the docs there is a section on converting a TileLayerRDD[K] to a RasterFrame. However, there is no documentation on how to make a MultibandTileLayerRDD[K] into a RasterFrame. It'd be useful if there was a section on this conversion as it's slightly different from the Tile case.