Whatsnew reorganization
Changes

1. Adds a highlights section (open to suggestions on what should be added / removed from here)
2. Reorders the enhcancements section to be
    1. Highlighted features
    2. Other features (sections)
    3. List of other new features (one-liners)
3. Reorders the "Backwards incompatible changes" section. We had two lists of "other incompatible changes" that I've merged into one at the end of the section.
4. Moved extension type changes out of the "Backwards incompatible changes section". 

cc @jorisvandenbossche 