DOC several .rst files missing from toctree

[qinhanmin2014]

Below are extracted from Circle CI log.

/home/ubuntu/scikit-learn/doc/datasets/kddcup99.rst: WARNING: document isn't included in any toctree
/home/ubuntu/scikit-learn/doc/documentation.rst: WARNING: document isn't included in any toctree
/home/ubuntu/scikit-learn/doc/modules/dp-derivation.rst: WARNING: document isn't included in any toctree
/home/ubuntu/scikit-learn/doc/themes/scikit-learn/static/ML_MAPS_README.rst: WARNING: document isn't included in any toctree

At least the first one and the third one deserves a fix.
This means that we can not get access to kddcup99.rst and dp-derivation.rst unless we type the url.

[zorroblue]

I would like to work on this :)