docs: add text about Pandas req for use of datasets

[rich-iannone]

This addresses a pyOpenSci reviewer comment. In the API reference section of the docs site, there is a note about a requirement for a Pandas installation when using any of the datasets from the datasets module.

Fixes: #529

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.79%. Comparing base (925fa41) to head (19e9b2b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #549   +/-   ##
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  Coverage   89.79%   89.79%           
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  Files          45       45           
  Lines        5321     5321           
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  Hits         4778     4778           
  Misses        543      543           

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[machow]

LGTM!