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.
I am not sure this section is still valid. On line 626 a bit above it is clearly said: "Scalar selection for ``[],.ix,.loc`` will always be label based". 
Also from the output it seems that there is no fallback anymore.

And I think we also discussed this in an issue some time ago.