Added links to useful discussions of groupby and SettingWithCopyWarning

[supern8ent]

Added links to useful discussions of groupby and SettingWithCopyWarning.

Did not run tests as only docs changed, but python make.py --single groupby and python make.py --single indexing successfully create the respective html files.

• closes DOC: nice links / examples for setting with copy & aggregation #17505
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #21916 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #21916   +/-   ##
=======================================
  Coverage   91.96%   91.96%           
=======================================
  Files         166      166           
  Lines       50323    50323           
=======================================
  Hits        46281    46281           
  Misses       4042     4042

Flag	Coverage Δ
#multiple	90.36% <ø> (ø)	⬆️
#single	42.23% <ø> (ø)	⬆️

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[jreback]

worth adding some text / examples from the blog as well?

@datapythonista

[datapythonista]

I'm unsure about adding many links to third parties in the documentation. If a blog is clearer than the documentation, I think we should improve the documentation as a preferred practice.

@supern8ent what do you think it's missing in the docs that those links clarify?

[supern8ent]

@datapythonista Honestly I was at my first sprint and just looking for a doc issue to squash. I agree that it's better to improve the documentation than link, but I can't answer your question (what specifically is missing) since it wasn't my idea in the first place.

@jreback I suggest rejecting the PR and clarifying issue 17505 a bit.

[jreback]

I thought some is the prose and examples in the blog were pretty clear. might be worth cribbing from it

[supern8ent]

Ok, just including links is not preferred. Better to use those works as a reference to improve pandas documentation.