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)
- 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!