DOC: clean-up 0.19.0 whatsnew file
WIP (it's actually not critical for rc, as we do link to the dev docs (which are updated after rc release) anyway)

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14176?src=pr) is 85.25% (diff: 100%)

> Merging [#14176](https://codecov.io/gh/pydata/pandas/pull/14176?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14176   diff @@
==========================================
  Files           140        139     -1   
  Lines         50549      50496    -53   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43091      43050    -41   
+ Misses         7458       7446    -12   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [9b7efd6...5c3ad38](https://codecov.io/gh/pydata/pandas/compare/9b7efd69b2e88f5019d480ea4d5719bfaa7fa26b...5c3ad383b150292096b8b94c092e74029a9c31b1?src=pr)
 @jreback Do you think of other things to include in the highlights?
 Going to merge this now, we can do further clean-up later
