DOC: add highlights and toc to whatsnew file for 0.23.0

[jorisvandenbossche]

@TomAugspurger in case you didn't do this yet

[jorisvandenbossche]

(don't merge yet, I will at once go through the full file as well)

[TomAugspurger]

Ah thanks, I had not.

[TomAugspurger]

@jorisvandenbossche can you fix #21051 (comment)

Should be "from 0.22.0", not "0.23.0" in release.rst.

[codecov]

Codecov Report

Merging #21061 into master will not change coverage.
The diff coverage is n/a.

@@           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)	⬆️
#single	41.88% <ø> (ø)	⬆️

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[TomAugspurger]

@jorisvandenbossche do you have more fixes to push here? This will be the last one once the CI finishes for #21062 and #21057.

[TomAugspurger]

OK @jorisvandenbossche just this one remaining :)

[jorisvandenbossche]

Merge away!

[TomAugspurger]

Thanks! can fixup docs later of course.