fix: add gitignore to custom folder instead of gitkeep

[ch-cbna]

See #3179

[jacquesfize]

Pas chaud de refaire un gitignore... (normalement) Il est possible de faire ça dans le .gitignore principale (voir #3179 (comment))

[jpm-cbna]

Il faudrait fusionner les différents commits de la branche pour faciliter la fusion.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.62%. Comparing base (3c2f64f) to head (64ad93f).
Report is 227 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3181      +/-   ##
===========================================
+ Coverage    78.50%   81.62%   +3.11%     
===========================================
  Files           89       86       -3     
  Lines         7208     6945     -263     
===========================================
+ Hits          5659     5669      +10     
+ Misses        1549     1276     -273     

Flag	Coverage Δ
pytest	81.62% <ø> (+3.11%)	⬆️

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[jpm-cbna]

Ok pour moi.