Merge "DTLS Client"and "DTLS Server" examples

I suggest to merge the two separate 'DTLS Client' and 'DTLS Server' examples in one common 'DTLS' example. CUrrently they show up and are documented as two independent examples, though the documentation says

Note: The DTLS client example is intended to be run alongside the DTLS server example.

From a users perspective, I think it would be better to document this as one example with two binaries. This would also take one space less in the 'featured'list in Qt Creator