[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 hate to be 'that guy', but I think that here you should use a '.. seealso' block :)

If by "that guy" you mean someone knowledgeable in rst/shinx, I don't see the shame in it!