DOC: reorg whatsnew file v0.16.0
No real edits, just some rearranging.

I added an automatic 'contents' in the beginning, so this gives all headers automatically, and in this way restricted the list in the beginning to just the highlights (then it can be copied exactly like this to the release notes in release.rst

@jreback ok for you?
 yep, lgtm.

will only be a 2-level menu on the left hand bar, but that's ok
