docs: Added minimal docstring sanity check

[fg-mindee]

This PR introduces the following modifications:

• added config for pydocstyle
• added deps in setup.py, updated CONTRIBUTING & Makefile
• added a CI job to run pydocstyle
• fixed docstrings that were posing problems

Any feedback is welcome!

[codecov]

Codecov Report

Merging #686 (c591119) into main (c2af6e9) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
+ Coverage   96.27%   96.36%   +0.08%     
==========================================
  Files         117      117              
  Lines        4540     4540              
==========================================
+ Hits         4371     4375       +4     
+ Misses        169      165       -4     

Flag	Coverage Δ
unittests	96.36% <ø> (+0.08%)	⬆️

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

Impacted Files	Coverage Δ
doctr/models/recognition/master/pytorch.py	95.58% <ø> (ø)
doctr/models/recognition/master/tensorflow.py	96.50% <ø> (ø)
doctr/utils/fonts.py	92.85% <ø> (ø)
doctr/utils/metrics.py	97.40% <ø> (ø)
doctr/utils/visualization.py	92.03% <ø> (ø)
doctr/models/detection/linknet/base.py	83.16% <0.00%> (-1.99%)	⬇️
...dels/detection/differentiable_binarization/base.py	92.77% <0.00%> (+0.55%)	⬆️
doctr/models/builder.py	99.12% <0.00%> (+2.63%)	⬆️
doctr/models/recognition/predictor/pytorch.py	96.96% <0.00%> (+6.06%)	⬆️

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[charlesmindee]

Thanks for that!