DOC: Fixes to the whatsnew structure
- [X] xref #24499
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Changes in this PR:
- Moving `releases.rst` to `whatsnew/index.rst`, which is more consistent with the new structure of the documentation
- Sorting the `whatsnew/index` from the newest to the oldest (was from newest to oldest for the major version, but inside them, the minor versions were sorted from oldest to newest, except for 0.23).

CC: @TomAugspurger 
