DOC: Fix table styling in main docs

[TomAugspurger]

When switching to nbsphinx, I modified the site's CSS so
that the converted notebook looks decent. This had some
unfortunate changes on tables elsewhere in the notebook.

This change fixes the headers to be left-aligned in the main site,
and right-aligned for the tables generated by df.style in the
nbsphinx-converted notebook.

xref #15581

Before:

After:

[jreback]

@TomAugspurger

When switching to nbsphinx, I modified the site's CSS so
that the converted notebook looks decent. This had some
unfortunate changes on tables elsewhere in the notebook.

what is this modification (I am needing to steal for another project).
nbsphinx is very nice btw.

[TomAugspurger]

what is this modification

Just the diff here. Rather than applying the text-align: right to all ths on the site, it only applies to ths with the col_heading class. .style includes col_heading in the generated HTML for headers.

[codecov]

Codecov Report

Merging #16145 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #16145      +/-   ##
==========================================
+ Coverage   90.83%   90.86%   +0.02%     
==========================================
  Files         159      159              
  Lines       50787    50787              
==========================================
+ Hits        46134    46146      +12     
+ Misses       4653     4641      -12

Flag	Coverage Δ
#multiple	88.64% <ø> (+0.02%)	⬆️
#single	40.32% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	65.35% <0%> (+1.81%)	⬆️

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[codecov]

Codecov Report

Merging #16145 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #16145      +/-   ##
==========================================
+ Coverage   90.83%   90.86%   +0.02%     
==========================================
  Files         159      159              
  Lines       50787    50787              
==========================================
+ Hits        46134    46146      +12     
+ Misses       4653     4641      -12

Flag	Coverage Δ
#multiple	88.64% <ø> (+0.02%)	⬆️
#single	40.32% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	65.35% <0%> (+1.81%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 61ca022...b37c518. Read the comment docs.