True when data looks like a Presentation API 3.0 Manifest.
A Collection is explicitly not one, even though it carries the same
@context. Detection used to accept anything with the P3 context, so a
Collection passed and its member Manifests were fed to canvasToSlide as if
they were Canvases — every member became a text-only slide, with no media, no
locations and no warning. within-style Collections are now handled by the
private collectionToStorymapData below, and anything else is rejected here.
True when a fetched document is an IIIF Presentation 3 manifest (accepted directly as a StoryMap source).