1: add the sequence diagram for the bluetooth example projects,bt_l2cap_server and bt_l2cap_client, which implenment the L2CAP connection.
2: add the discription for the sequence diagram.
This updates the minimal supported version of CMake to 3.16, which in turn enables us to use more CMake features and have a cleaner build system.
This is the version that provides most new features and also the one we use in our latest docker image for CI.
