Data usage vocabulary examples updated

As discussed in Zagreb, a decision was made to replace the 3 examples with
6 examples that 1) use the bus timetable example from the best practices
document 2) Help illustrate aspects of the DUV.

http://w3c.github.io/dwbp/vocab-du.html

The 6 examples have been integrated as citations in the vocabulary overview
section.

Kind regards,

Eric S