Document still/held image support

Sometimes a clip's media is a single image, held for some duration. We have found that folks using OTIO are surprised by this. They seem to expect that a clip's media_reference is always a movie file, or an ImageSequenceReference, but that is not a requirement.

We should document this use case, and provide OTIO samples with clips that point to still images.

Start by locating the existing OTIO documentation and sample material, then review how clips, media_reference, movie files, and ImageSequenceReference are currently described. Document that a clip may reference a single still image held for a duration, and add samples showing clips that point to still images.

Written by the indexing model from the issue text.