Use toctree in slide-index.rst to ensure all documents are listed
in toctree to avoid sphinx warnings.
Exclude associate-guides from HTML build. Also remove 'source' from
doc/training-guides/conf.py as it does not exists in doc/training-guide.
Ensure associate-guide/notes files are in toctree to avoid sphinx warnings.
These files are referred to by including them, so hidden toctrees are used.
Also specifies sphinx-build -W option to detect warnings.
Change-Id: I298202e3304d02f362cc4bf9cc58686857040fd8
5e0f091688