Add debug mode to TOC

This is coming from the oniro repository but the TOC is redefined in
docs leaving that document dangling.

This also fixes a build warning treated as an error:

Warning, treated as error:
[...]/docs/oniro/debug-mode.rst:document isn't included in any toctree
make: *** [Makefile:16: build] Error

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

index.rst

@@ -77,7 +77,8 @@ daily life.
    :caption: Troubleshoot
    :maxdepth: 2
 
-   oniro/fallback-devices-support 
+   oniro/fallback-devices-support
+   oniro/debug-mode
 
 .. toctree::
    :caption: Contribute