a "mainEntity" of something is always an "about" value too - express as subPropertyOf

[danbri]

Noting @nicolastorzec 's concern in #1525 that we have "mainEntity", "about" and also "mentions" properties, this would clarify a systematic relationship between "mainEntity" and "about". The "about" property can be used in a broad subject/topic oriented way (e.g. with SKOS URLs for concepts like "social policy"), but also to indicate specific identifiable entities such as people/places/things.

If we say

• some_doc mainEntity some_entity, let's have that imply
• some_doc about some_entity.

This corresponds with the established meaning of the property.

If we later go ahead with the subjectOf proposal (i.e. a property inverseOf 'about'). #1670 then we'd want to handle the inverseOf mainEntity too, i.e. subjectOf would be subPropertyOf mainEntityOfPage.