libbladeRF: Added sync rx/tx example to doc directory

The existing bladerf_sync_rx() snippet in the libbladeRF API docs has
been replaced by a more complete snippet in libbladeRF/doc/examples.

From now on, major snippets and examples to be included in
Doxygen-generated documentation shall now be placed in the
libbladeRF/doc/examples directory, and must be buildable and testable.

The Doxygen "snippet" command to shall be used to include the relevant
code from the example program.