DOC/REL: clean-up and restructure v0.15.0 whatsnew file (GH8477)
Closes #8477 

What did I change:
- some general clean-up (blank lines, ..)
- put the different new feature sections in front (as also in the highlights section)
- the 2-level menu structure as discussed in #8477 
- API section: move some entries to the 'enhancements' section (entries that were not 'real' (in sense of breaking) API changes):
  - describe keywords include/exclude
  - new 'level' keyword for Index.set_names/set_levels
  - new 'level' keyword for Index isin
  - histogram/boxplot also via `plot`
  - index new methods duplicated and drop_duplicates

Still work in progress!

you are going to incorporate this in 0.15.0 release? (ok by me)
 @jreback it can just be merged here (so it is in history and in the repo, but only after the 0.15 tag. So not really in the 0.15 release, but not a problem I think). I can just update the docs by checking out the 0.15 tag and applying this patch but without committing.
 @jorisvandenbossche yep that's what I was going to do.

ok, will announce (everything is posted already, so need to do it). Then merge / change docs as needed.

thanks!
 yes, go ahead with announcing, not need to wait on this (everything is already there, it is just a bit restructuring). And thank you!
 @jorisvandenbossche merge this when ready (then will close 0.15.0 milestone)
 @jorisvandenbossche isn't this done with released docs? (or you just need to merge this in).

prob need to change the template for 0.15.1 a bit as well
 yes, I will try to finish this tomorrow and then merge (most changes are already in the online docs now, only the last ones not yet, but will then rebuild the released docs tomorrow as well). And will also model the 0.15.1 whatsnew file accordingly.
 @jorisvandenbossche perfect thxs
 OK, finished this up. Updated docs that reflect these changes here are also online.

@jreback  and I added a wiki page on building/uploading the docs for a release: https://github.com/pydata/pandas/wiki/Building-and-uploading-the-official-documentation
