refactor: Relocated test/ to tests/ and made contribution process easier

[fg-mindee]

This PR introduces the following modifications:

• moved test/ to tests/
• added new extras in setup.py
• added Makefile for easier development procedure (from the repo folder, running make test will run all the CI unittests, make quality will run all style checks, and make docs will build the documentation)
• updates CONTRIBUTING, CI accordingly
• added a unittest for dependency consistency check (checking that we have the exact same version constraint between setup.py and all requirements.txt)

Any feedback is welcome!

[charlesmindee]

thanks!

[codecov]

Codecov Report

Merging #598 (3e41682) into main (f537551) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #598   +/-   ##
=======================================
  Coverage   96.03%   96.03%           
=======================================
  Files         110      110           
  Lines        4264     4265    +1     
=======================================
+ Hits         4095     4096    +1     
  Misses        169      169           

Flag	Coverage Δ
unittests	96.03% <ø> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
doctr/datasets/detection.py	96.96% <0.00%> (ø)
doctr/datasets/recognition.py	92.00% <0.00%> (ø)
doctr/models/predictor/tensorflow.py	100.00% <0.00%> (ø)

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[charlesmindee]

Thanks!