DOC: changes to use code-block declaration

[SarahJessica]

• closes DOC: changes to use code-block declaration #19400
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[TomAugspurger]

Thanks,

Looks like there was an extra file commited "doc/test.json". Could you delete it and then

git add doc/test.json
git commit -m "removed file"
git push

Then we'll get this merged.

[codecov]

Codecov Report

Merging #19400 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #19400      +/-   ##
==========================================
- Coverage   91.65%   91.62%   -0.03%     
==========================================
  Files         150      150              
  Lines       48726    48726              
==========================================
- Hits        44658    44646      -12     
- Misses       4068     4080      +12

Flag	Coverage Δ
#multiple	89.99% <ø> (-0.03%)	⬇️
#single	41.75% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	65.22% <0%> (-1.74%)	⬇️

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