Minimum number of properties not mentioned in the property reference

[javagl]

The mesh.primitive.attributes schema defines

"minProperties": 1,

This is not mentioned in the mesh.primitive.attributes property reference.

(There might be further instances of this, just giving one example here)

(An aside: For arrays, there is minItems/maxItems, and this is summarized somewhat implicitly in the "array size suffixes" like [1-*]. For dictionaries, there is minProperties/maxProperties, and this is not handled by wetzel yet)

[gurdep]

thanks @javagl for raising this. I think, the condition on the minProperties for mesh.primitives.attributes must be mentioned in the property reference as well.

This would address the question 3 in #2153