CLN: Index imports and 0.23.1 whatsnew

[mroeschke]

xref @jreback #21216 (comment) and cleaned up some formatting in the 0.23.1 whatsnew where GH issues references didn't hyperlink

[codecov]

Codecov Report

Merging #21490 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #21490      +/-   ##
==========================================
+ Coverage   91.89%   91.89%   +<.01%     
==========================================
  Files         153      153              
  Lines       49604    49604              
==========================================
+ Hits        45584    45586       +2     
+ Misses       4020     4018       -2

Flag	Coverage Δ
#multiple	90.3% <100%> (ø)	⬆️
#single	41.88% <83.33%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/indexes/base.py	96.62% <100%> (ø)	⬆️
pandas/util/testing.py	84.81% <0%> (+0.2%)	⬆️

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[jreback]

thanks @mroeschke