DOC: reorg indexing docs
DOC: created advanced indexing section
DOC: add internals section (GH4082)

Mostly just splitting up the indexing docs, but I reorded / revised the Multiindex sections
a bit to avoid duplication / make more clear.

@jorisvandenbossche pls have a look
 ![pandas powerful python data analysis toolkit pandas 0 14 1-354-g12248ff documentation](https://cloud.githubusercontent.com/assets/953992/4180278/21517dca-36f1-11e4-94e3-c3abeabf991c.png)
 ![multiindex advanced indexing pandas 0 14 1-354-g12248ff documentation](https://cloud.githubusercontent.com/assets/953992/4180279/2b887668-36f1-11e4-9c0b-0ab32eca28a6.png)
 +1 on splitting it up a bit. I have wanted to do that for the rest of the docs also. I personally think it would even be better to also have a more nested hierarchy, but the current html theme does not really support that.

I will try to take a more detailed look tomorrow on the docs itself.
 @jorisvandenbossche gr8 thanks.
 lmk after u have reviewed this
 @jreback I went through it, some comments (probably mostly on the content that was already there but just moved, but a good occasion to check that). For the rest, certainly fine to merge!
