Add examples and documentation for new UUID behavior

We should an FAQ entry as well as a full example page that documents the new UUID behavior (and how this behavior is going to change in PyMongo 4.0). Specifically, we need to document:

• The behavior of the UNSPECIFIED UuidRepresentation. This should be accompanied by an example of how a user can manually coerce a subtype 3 or 4 Binary value returned by PyMongo to a native UUID.
• The new default value of UuidRepresentation that will be used starting in PyMongo 4.0.
• How the behavior of existing UuidRepresentations will change between PyMongo 3.x to PyMongo 4.0.