DOC: Add whatsnew for 2.3.0
We are probably not going to need all sections for the 2.3.0 release (and need some custom sections for the upcoming changes), but just adding our standard template and can clean up later.
```bash
 /home/runner/work/pandas/pandas/doc/source/whatsnew/v3.0.0.rst:4: WARNING: duplicate label whatsnew_230, other instance in /home/runner/work/pandas/pandas/doc/source/whatsnew/v2.3.0.rst
``` Thanks @jorisvandenbossche  (cc @WillAyd this needs to be backported as well)