DOC: Rework the advanced indexing documentation.

Mostly makes the advanced indexing doc much more example based,
and prominently mentions the np.ix_ function.
Some subtleties (some of which are new) are also mentioned.

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I am not a big doc writer, but the advanced indexing documentation seemed a bit too technical and few examples. I am open to revaminping this a few times if it is in the right direction. Some of the details noted are only quite correct with my indexing rework applied.