DOC: clean up 0.14.1 whatsnew file

- removed the sections with no entries
- removed whitespace
- moved some entries from API-changes to Enhancements/Big fixes when it was not really a backwards incompatible change or if it was not clear what the relevant change was for users (and for the offsets normalize issue I rewrote it)

Highlights should still be written to include here and in the release.rst