DOC: fix for tons of whitespace on the user_guide page of new dev-webpage

fix sidebar button size staying constant and leaving a mountain of white-space due to expanding/collapsing toctree

As mentioned in #2335

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Compare [current User Guide](http://scikit-learn.org/dev/user_guide.html) to [my fix](http://jaquesgrobler.github.io/online-sklearn-build/user_guide.html)