doc: Add information on how to reproduce the Reference Stack's use-cases

Add instructions on how to reproduce the Heterogeneous Inter-processor
Communication use-case, which entails:

 * Accessing both the Primary Compute and the Safety Island terminals with
   telnet.
 * Pinging the Safety Island from the Primary Compute.
 * Pinging the Primary Compute from the Safety Island.
 * Stress testing the following connections:
   * Primary Compute as a UDP client, Safety Island as a UDP server.
   * Primary Compute as a TCP client, Safety Island as a UDP server.
   * Primary Compute as a UDP server, Safety Island as a UDP client.
   * Primary Compute as a TCP server, Safety Island as a TCP client.

Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
Issue-Id: SCM-5876
Change-Id: Ic0a78168165d1bec1d8c7e64c8d00955bfa3c631