doc: add VSC instructions for Thread Test Harness integration

Added instructions for using VSC as an alternative to
command line when building the CLI sample for
Thread certification using the Thread Test Harness.

Signed-off-by: Wille Backman <[email protected]>

Author: wiba-nordic

doc/nrf/ug_thread_certification.rst

@@ -66,10 +66,17 @@ Complete the following steps to prepare for the certification tests:
 
    The :ref:`ot_cli_sample` sample is used as a base, modified with the :file:`harness/overlay-cert.conf` overlay file.
 
-   .. code-block::
+   * If building on the command line, use the following command:
 
-         cd ncs/nrf/samples/openthread/cli/
-         west build -b nrf52840dk_nrf52840 -- -DOVERLAY_CONFIG=harness/overlay-cert.conf -DCONFIG_OPENTHREAD_LIBRARY_1_1=y
+     .. code-block::
+
+        cd ncs/nrf/samples/openthread/cli/
+        west build -b nrf52840dk_nrf52840 -- -DOVERLAY_CONFIG=harness/overlay-cert.conf -DCONFIG_OPENTHREAD_LIBRARY_1_1=y
+
+   * If building using Visual Studio Code, you must first `create the application <Creating an application_>`_ using the CLI sample, and then `build the application <Building an application_>`_.
+     Select the :file:`harness/overlay-cert.conf` overlay file in the :guilabel:`Kconfig fragment` drop-down menu and add ``CONFIG_OPENTHREAD_LIBRARY_1_1=y`` to the :guilabel:`Additional CMake arguments` text field.
+
+     If the overlay file is not visible in the drop-down menu, navigate to :file:`ncs/nrf/samples/openthread/cli/harness/` and copy the :file:`overlay-cert.conf` file to :file:`ncs/nrf/samples/openthread/cli/` and try again.
 
    .. note::
       The configuration option selects the precompiled OpenThread libraries.