doc: shell: Add section about PuTTY RTT configuration

Add section which explains how to configure PuTTY to use RTT
shell there with all shell features. There is an alternative
approach which also works but it does not allow to use all
shell features.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>

doc/services/shell/index.rst

@@ -122,6 +122,30 @@ To configure Segger RTT backend, add the following configurations to your build:
 Details on additional configuration settings are captured in:
 :zephyr_file:`samples/subsys/shell/shell_module/prj_minimal_rtt.conf`.
 
+.. _shell_rtt_putty:
+
+Using PuTTY
+-----------
+
+Use following procedure:
+
+* Open debug session and continue running the application.
+
+  .. code-block:: none
+
+     west attach
+
+* Open ``PuTTY``. Use telnet port 19021 and specific Terminal configuration. Set ``Local echo``
+  to ``Force off`` and ``Local line editing`` to ``Force off`` (see image below).
+
+
+.. image:: images/putty_rtt.png
+      :align: center
+      :alt: RTT PuTTY terminal configuration.
+
+* Now you should have a network connection to RTT that will let you enter input
+  to the shell.
+
 Connecting to Segger RTT via TCP (on macOS, for example)
 --------------------------------------------------------
 
@@ -143,7 +167,9 @@ procedure:
      nc localhost 19021
 
 * Now you should have a network connection to RTT that will let you enter input
-  to the shell.
+  to the shell. However, contrary to `PuTTY <shell_rtt_putty_>`_ some features like
+  ``Tab`` completion do not work.
+
 
 Commands
 ********