[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.
@GaelVaroquaux, if the changes in 42097f2bc67512464c5a0305623d61f9941c6056 are satisfactory in terms of linking pipelines into model selection, do I have your +1? (and I assume one +1 should suffice for this PR)