Most warnings are "document isn't included in any toctree".
This is because documents are not listed in any toctree.
In the upstream training document, explicit toctree looks unnecessary,
so this commit adds a hidden toctree to cover all unlisted documents.
It makes these documents reachable from standard navigation (prev/next).
doc/upstream-training/README.rst is not intended to link from the document.
Add README.rst to exclude_patterns in conf.py.
Also specifies sphinx-build -W option to detect warnings.
Change-Id: I42750cde20a35342dce7857e275f7fa27211236b
82731ddab0