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