plot_dist: miscelaneous fixes

[aloctavodia]

Closes #238.

This PR fix the following issues

• Add proper reference to Kruschke's book
• Change default color "gray" to a darker hue, for better contrast.
• Correctly positions the point_estimate_text when densities are committed

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.38%. Comparing base (9c7b553) to head (ff07514).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #243   +/-   ##
=======================================
  Coverage   68.38%   68.38%           
=======================================
  Files          40       40           
  Lines        4552     4552           
=======================================
  Hits         3113     3113           
  Misses       1439     1439           

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