Add updated documentation from the stix schemas documentation branch

To do this I:

* Created a documentation updates branch in the STIX schemas
* Added documentation from the whitepaper for the core constructs (minus Observable, which is CybOX)
* Updated stix_schema_spy to allow preloading schemas from a custom directory
* Added a git remote to the documentation updates branch in the STIX schemas
* Updated the data model page template a little bit to be more clear when there's both long documentation and suggested practices
* Regenerated data model pages