Documentation changes

[percygautam]

Description

Modified the extend_xr_method to include docstring and examples. In continuation with the #1223 and #1254 and #1300. Related to #1066 .

Checklist

• Follows official PR format
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[percygautam]

I have added examples for custom extended methods and will start adding examples for extended methods. It will be helpful if it can get reviewed once as I continue adding examples.

[codecov]

Codecov Report

Merging #1338 (8567170) into master (25ce6c4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1338   +/-   ##
=======================================
  Coverage   91.92%   91.92%           
=======================================
  Files         105      105           
  Lines       11250    11252    +2     
=======================================
+ Hits        10341    10343    +2     
  Misses        909      909           

Impacted Files	Coverage Δ
arviz/data/base.py	97.74% <100.00%> (+0.03%)	⬆️
arviz/data/inference_data.py	85.81% <100.00%> (ø)

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