DOC: add highlights and toc to whatsnew file for 0.23.0
@TomAugspurger in case you didn't do this yet
(don't merge yet, I will at once go through the full file as well) Ah thanks, I had not. @jorisvandenbossche can you fix https://github.com/pandas-dev/pandas/pull/21051#issuecomment-389226304

Should be "from 0.22.0", not "0.23.0"  in `release.rst`. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21061?src=pr&el=h1) Report
> Merging [#21061](https://codecov.io/gh/pandas-dev/pandas/pull/21061?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/dfb265166c51fa20c473fbb0726f11442042ee03?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21061/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21061?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21061   +/-   ##
=======================================
  Coverage   91.82%   91.82%           
=======================================
  Files         153      153           
  Lines       49503    49503           
=======================================
  Hits        45458    45458           
  Misses       4045     4045
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.22% <ø> (ø)` | :arrow_up: |
| #single | `41.88% <ø> (ø)` | :arrow_up: |


------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21061?src=pr&el=footer). Last update [dfb2651...f269ebe](https://codecov.io/gh/pandas-dev/pandas/pull/21061?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jorisvandenbossche do you have more fixes to push here? This will be the last one once the CI finishes for #21062 and #21057. OK @jorisvandenbossche just this one remaining :) Merge away! Thanks! can fixup docs later of course.