Make Jitter do something by default

[mwaskom]

This adds a small amount of width jitter (i.e., jitter along the orientation axis proportional to the spacing between marks) by default:

(
    so.Plot(penguins, "species", "body_mass_g")
    .add(so.Dots(), so.Jitter())
)

The default jitter is disabled if any of the other jitter parameters (x or y) are specified.

Additionally, added API examples.

[codecov]

Codecov Report

Merging #3066 (e159427) into master (aa56714) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3066   +/-   ##
=======================================
  Coverage   98.38%   98.38%           
=======================================
  Files          74       74           
  Lines       23635    23648   +13     
=======================================
+ Hits        23254    23267   +13     
  Misses        381      381           

Impacted Files	Coverage Δ
seaborn/_core/moves.py	100.00% <100.00%> (ø)
seaborn/_core/plot.py	99.31% <100.00%> (-0.01%)	⬇️
seaborn/_core/typing.py	100.00% <100.00%> (ø)
tests/_core/test_moves.py	100.00% <100.00%> (ø)