add observed_rug argument to plot_ppc

[aloctavodia]

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

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📚 Documentation preview 📚: https://arviz--2161.org.readthedocs.build/en/2161/

[codecov]

Codecov Report

Merging #2161 (0733f3a) into main (24e66c3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2161   +/-   ##
=======================================
  Coverage   90.67%   90.68%           
=======================================
  Files         120      120           
  Lines       12667    12678   +11     
=======================================
+ Hits        11486    11497   +11     
  Misses       1181     1181           

Impacted Files	Coverage Δ
arviz/plots/ppcplot.py	89.18% <ø> (ø)
arviz/plots/backends/bokeh/ppcplot.py	97.64% <100.00%> (+0.13%)	⬆️
arviz/plots/backends/matplotlib/ppcplot.py	97.64% <100.00%> (+0.02%)	⬆️

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