Update docs to use Apptainer instead of Singularity

[clelange]

By now most sites will have moved to Apptainer and even though singularity is typically soft-linked to apptainer, using SINGULARITY_CACHEDIR instead of APPTAINER_CACHEDIR will lead to a warning, so updating the docs is better.

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📚 Documentation preview 📚: https://hepdata-lib--244.org.readthedocs.build/en/244/

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f194e14) 87.71% compared to head (db84937) 87.71%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #244   +/-   ##
=======================================
  Coverage   87.71%   87.71%           
=======================================
  Files           5        5           
  Lines        1083     1083           
  Branches      231      231           
=======================================
  Hits          950      950           
  Misses         93       93           
  Partials       40       40           

Flag	Coverage Δ
unittests-3.10	87.71% <ø> (ø)
unittests-3.6	87.40% <ø> (ø)
unittests-3.7	87.40% <ø> (ø)
unittests-3.8	87.53% <ø> (ø)
unittests-3.9	87.53% <ø> (ø)

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