feat: added DocArtefacts dataset

[SiddhantBahuguna]

This PR introduces the following modifications:

• Implemented class wrapper for artefact detection dataset

Any feedback is welcome!

[fg-mindee]

Thanks for the PR! Looks to me like we're missing two things:

• Added the corresponding entry in the documentation
• Adding corresponding unittest

[codecov]

Codecov Report

Merging #583 (058138a) into main (c882689) will decrease coverage by 0.06%.
The diff coverage is 93.54%.

❗ Current head 058138a differs from pull request most recent head 72f54db. Consider uploading reports for the commit 72f54db to get more accurate results

@@            Coverage Diff             @@
##             main     #583      +/-   ##
==========================================
- Coverage   96.05%   95.98%   -0.07%     
==========================================
  Files         109      110       +1     
  Lines        4233     4264      +31     
==========================================
+ Hits         4066     4093      +27     
- Misses        167      171       +4     

Flag	Coverage Δ
unittests	95.98% <93.54%> (-0.07%)	⬇️

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

Impacted Files	Coverage Δ
doctr/datasets/doc_artefacts.py	93.33% <93.33%> (ø)
doctr/datasets/__init__.py	100.00% <100.00%> (ø)
doctr/datasets/funsd.py	93.75% <0.00%> (-3.13%)	⬇️
doctr/datasets/sroie.py	92.30% <0.00%> (-2.57%)	⬇️
doctr/datasets/cord.py	95.00% <0.00%> (-2.50%)	⬇️
...dels/detection/differentiable_binarization/base.py	91.82% <0.00%> (+0.62%)	⬆️

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[fg-mindee]

Thanks!