[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 can understand the logic for moving the pipeline docs to the transformation part. However, I think that we need to add references to the pipeline section in different sections of the model-selection chapter (probably using the 'topic' rest directive).