docs: add Debug Mode documentation

Add basic documentation for the debug mode.

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>

docs/debug-mode.rst

+.. SPDX-FileCopyrightText: Huawei Inc.
+..
+.. SPDX-License-Identifier: CC-BY-4.0
+
+.. include:: definitions.rst
+
+.. _DebugMode:
+
+Debug Mode
+##########
+
+In some situations, you may want to compile an image with some additional tools
+and features for easier debugging. For this reason, |main_project_name|
+offers the debug mode.
+
+Images built with the `debug mode` enabled contain additional tools and allow
+to log in as root (which is disabled in the default images).
+
+To enable the `debug mode`, edit your ``local.conf`` and add:
+
+.. code-block:: console
+
+    INHERIT += "oniro-debug-linux"
+
+For convenience, the section is already present commented-out in the template
+and you may uncoment it.
+
+When done, rebuild your image as usual. To go back to the production mode,
+comment the line out again and rebuild your image.
+
+.. Note::
+
+   Debug mode images are unsupported by the project and do not take part
+   of the release. They are provided for convenience only.

docs/index.rst

@@ -40,4 +40,5 @@ Build System Guide
    :caption: Troubleshoot
    :maxdepth: 2
 
-   fallback-devices-support 
+   fallback-devices-support
+   debug-mode