Convert color palette docstrings to notebooks

[mwaskom]

This completes the removal of doctest-style examples from the seaborn codebase, so testing infrastructure complexity distinguishing between unit/doc tests is also removed. Closes #2110

Additionally:

• make various docstring improvements
• provide better feedback for the colormap widgets when ipywidgets is not installed.

[codecov]

Codecov Report

Merging #3034 (d7fa812) into master (54c36b7) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3034      +/-   ##
==========================================
- Coverage   98.32%   98.25%   -0.08%     
==========================================
  Files          69       69              
  Lines       23241    23270      +29     
==========================================
+ Hits        22852    22864      +12     
- Misses        389      406      +17     

Impacted Files	Coverage Δ
seaborn/rcmod.py	100.00% <ø> (ø)
seaborn/palettes.py	99.60% <100.00%> (+0.03%)	⬆️
tests/test_palettes.py	100.00% <100.00%> (ø)
seaborn/objects.py	0.00% <0.00%> (-100.00%)	⬇️
seaborn/_docstrings.py	85.36% <0.00%> (-14.64%)	⬇️
seaborn/_compat.py	46.46% <0.00%> (+1.01%)	⬆️