Added an example and improved docstring for load_sample

[kennethshsu]

What does this pull request do?

Improved the doctoring and added an example

Relevent Github ticket(s)?

#704

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Note

Low Risk
Low risk since changes are documentation-only and do not alter load_sample behavior or data loading logic.

Overview
Updates the load_sample docstring to clarify that dataset keys are case-insensitive, enumerate commonly used and all available built-in sample datasets, and include a Sphinx .. testcode:: example showing how to load raa and the expected output.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.12%. Comparing base (6ea3577) to head (aec82e9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #715   +/-   ##
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  Coverage   85.12%   85.12%           
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  Files          85       85           
  Lines        4909     4909           
  Branches      629      629           
=======================================
  Hits         4179     4179           
  Misses        521      521           
  Partials      209      209           

Flag	Coverage Δ
unittests	85.12% <ø> (ø)

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[kennethshsu]

@genedan you can hold off on this too, I'll need to update before this can be reviewed and merged.