Add documentation/examples for reprojecting RGB images

It's now possible to reproject RGB images with AVM meta-data, but we don't currently document it outside of the docstrings. We should add proper documentation for this. This includes but is not limited to the case of reprojecting to HiPS.

We should also make sure this gives a nice error if there is no or invalid AVM metadata.

Start with the existing docstrings for RGB-image reprojection and the current HiPS-related documentation or examples. Check how missing or invalid AVM metadata behaves, then add documentation covering RGB and HiPS reprojection and ensure the failure message is clear.

Written by the indexing model from the issue text.