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!
