Improved description of seaborn

[mwaskom]

The pandas docs send a surprising amount of traffic towards seaborn, but
I've never thought that the existing description communicates the
goals/advantages of seaborn all that well. Please consider this revised
copy.

I also swapped the ordering of seaborn/py-ggplot, because it looks like
that package has stalled out a bit, but I'm happy to revert the
reordering if it bothers people.

[jreback]

lgtm. @TomAugspurger

[codecov]

Codecov Report

Merging #18495 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18495      +/-   ##
==========================================
+ Coverage    91.3%   91.32%   +0.02%     
==========================================
  Files         163      163              
  Lines       49781    49781              
==========================================
+ Hits        45451    45463      +12     
+ Misses       4330     4318      -12

Flag	Coverage Δ
#multiple	89.12% <ø> (+0.02%)	⬆️
#single	40.72% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	65.25% <0%> (+1.81%)	⬆️

---

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[codecov]

Codecov Report

Merging #18495 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18495      +/-   ##
==========================================
+ Coverage    91.3%   91.32%   +0.02%     
==========================================
  Files         163      163              
  Lines       49781    49781              
==========================================
+ Hits        45451    45463      +12     
+ Misses       4330     4318      -12

Flag	Coverage Δ
#multiple	89.12% <ø> (+0.02%)	⬆️
#single	40.72% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	65.25% <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 38f41e6...ba036c5. Read the comment docs.

[jreback]

thanks @mwaskom

[jorisvandenbossche]

We should actually also update the ggplot -> ggpy, or rather add the much more active https://github.com/has2k1/plotnine