Add ProSelecta example to add_additional_resource in docs

[GraemeWatt]

This PR updates the hepdata_lib documentation to mention some new functionality recently added to HEPData, where records with an additional resource file with type: ProSelecta are flagged with a "NUISANCE" badge (see NUISANCE section of submission documentation).

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📚 Documentation preview 📚: https://hepdata-lib--270.org.readthedocs.build/en/270/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.42%. Comparing base (24863e9) to head (9ad4174).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #270   +/-   ##
=======================================
  Coverage   90.42%   90.42%           
=======================================
  Files           5        5           
  Lines        1117     1117           
  Branches      254      254           
=======================================
  Hits         1010     1010           
  Misses         78       78           
  Partials       29       29           

Flag	Coverage Δ
unittests-3.10	90.42% <ø> (ø)
unittests-3.11	90.42% <ø> (ø)
unittests-3.6	90.14% <ø> (ø)
unittests-3.7	90.14% <ø> (ø)
unittests-3.8	90.24% <ø> (ø)
unittests-3.9	90.24% <ø> (ø)

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