Document ATT&CK types in the STIX data

[isaisabel]

As a user of the ATT&CK dataset, I want to be able to populate my tools and UIs with information about the ATT&CK data model.

Currently, there's a good deal of information about the representation of ATT&CK types such as groups which is not encoded in the STIX, e.g the groups introduction found on the ATT&CK website. It would be advantageous to users if we could encode this information somewhere in the ATT&CK STIX representation so that user interfaces and tools could provide this context when warranted.