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>
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