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.
You don't really say how using `IndexSlice` is a shortcut (you can observe it from the examples, but maybe also saying explicitely: "to be able to use ``:`` instead of `slice(None)`"