index: add default-password doc from oniro

Reference the recently added document in the oniro repo.

Signed-off-by: Marta Rybczynska <marta.rybczynska@linaro.org>

index.rst

@@ -89,6 +89,7 @@ daily life.
    oniro/fallback-devices-support
    oniro/debug-mode
    oniro/update-tool
+   oniro/default-password
 
 .. toctree::
    :caption: Contribute