feat: Added file existence check in DetectionDataset

[fg-mindee]

This PR introduces the following modifications:

• adds the file existence check in doctr.datasets.DetectionDataset
• refactors RecognitionDataset constructor
• adds unittest for file existence check

Any feedback is welcome!

[codecov]

Codecov Report

Merging #672 (03414c8) into main (1dad108) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #672      +/-   ##
==========================================
+ Coverage   96.17%   96.24%   +0.06%     
==========================================
  Files         117      117              
  Lines        4472     4471       -1     
==========================================
+ Hits         4301     4303       +2     
+ Misses        171      168       -3     

Flag	Coverage Δ
unittests	96.24% <100.00%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
doctr/datasets/cord.py	97.43% <ø> (ø)
doctr/datasets/doc_artefacts.py	94.11% <ø> (ø)
doctr/datasets/funsd.py	96.87% <ø> (ø)
doctr/datasets/sroie.py	94.87% <ø> (ø)
doctr/datasets/detection.py	97.05% <100.00%> (+0.18%)	⬆️
doctr/datasets/recognition.py	81.81% <100.00%> (+5.81%)	⬆️
doctr/datasets/ocr.py	92.00% <0.00%> (+4.00%)	⬆️

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[charlesmindee]

Thanks!