Add more details on Apptainer usage to setup/getting started docs

[clelange]

This should also improve consistency between README and general docs.

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📚 Documentation preview 📚: https://hepdata-lib--268.org.readthedocs.build/en/268/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.42%. Comparing base (11bc398) to head (f520a20).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #268   +/-   ##
=======================================
  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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