plot_ppc related fixes

[OriolAbril]

Description

Modify plot_ppc default labeling and several plot_ppc fixes, mainly working properly when chain and draw aren't the first dimensions positionally

Checklist

• [x] Follows official PR format
• [x] Includes new or updated tests to prevent regression on the fix
• [x] Code style correct (follows pylint and black guidelines)
• [x] Changes are listed in changelog

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:books: Documentation preview :books:: https://arviz--2283.org.readthedocs.build/en/2283/

[codecov[bot]]

Codecov Report

Merging #2283 (1b06391) into main (4557004) will increase coverage by 0.07%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2283      +/-   ##
==========================================
+ Coverage   86.71%   86.78%   +0.07%     
==========================================
  Files         122      122              
  Lines       12459    12460       +1     
==========================================
+ Hits        10804    10814      +10     
+ Misses       1655     1646       -9     

Files	Coverage Δ
arviz/labels.py	93.67% <100.00%> (+0.16%)	:arrow_up:
arviz/plots/backends/matplotlib/ppcplot.py	97.63% <ø> (-0.02%)	:arrow_down:
arviz/plots/ppcplot.py	89.33% <ø> (ø)

... and 2 files with indirect coverage changes