[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.
> > +    chained in one step. See :ref:`pipeline`.

> Log says why:

> WARNING: undefined label: pipeline (if the link has no caption the label must precede a section header)

This probably means that the label used is wrong, so we need to find what
the actual label is and fix it. I think that that you are not responsible
for this, as it as wrong and you just moved it. However it would be great
if you could fix it. Thanks!