Enhance docstrings and examples for improved clarity for many estimators

[kennethshsu]

Summary of Changes

• Improved docstrings and examples for many estimators: Chainladder, BF, CapeCod, Benktander, ExpectedLoss
• Added Expected Loss to the API reference, it was missing for some reason
• Added a little checkbox to the PR template to help with code tests

Related GitHub Issue(s)

#704

Additional Context for Reviewers

None

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Note

Low Risk
Changes are limited to documentation/docstrings and contributor workflow guidance; runtime behavior is not modified. Main risk is doctest/example output drift causing docs build failures.

Overview
Improves estimator documentation and doctest examples for Benktander, BornhuetterFerguson, CapeCod, and ExpectedLoss, including clearer parameter guidance (e.g., n_iters behavior and apriori vs sample_weight) and refreshed sample-based outputs.

Documentation plumbing updates: adds ExpectedLoss to the API reference (and generated rst stub), updates contributing guidance and PR template with a checkbox and explicit commands to run pytest and rebuild docs with doctests.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.94%. Comparing base (f1fd507) to head (0d15ebc).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #747   +/-   ##
=======================================
  Coverage   85.94%   85.94%           
=======================================
  Files          85       85           
  Lines        4924     4924           
  Branches      637      637           
=======================================
  Hits         4232     4232           
  Misses        491      491           
  Partials      201      201           

Flag	Coverage Δ
unittests	85.94% <ø> (ø)

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