[MRG] DOC restructure data transformation user guide

This PR restructures the data transformations narrative documentation, most notably including `Pipeline` and `FeatureUnion`, as per #1383. It also separates discussion of preprocessing into more appropriate sections, and gives an introduction to the chapter as a whole.

It might yet be beneficial to move random projections as a whole into the unsupervised dimensionality reduction section.
I usually like to make the titles as easy as possible to understand, independently of the background of the user.

I think that I would maybe call this "Transforming the prediction target ('y')".