feat: Added a dual framework availability checker

[fg-mindee]

Following up on #261, this PR introduces the following modifications:

• added a dynamic framework selection module (which can then be used to dynamically import the correct modules in __init__.py)
• updated all installation instructions
• updated installation commands
• added a unittest

Any feedback is welcome!

[codecov]

Codecov Report

Merging #306 (7d01110) into main (f6b0ff9) will decrease coverage by 0.58%.
The diff coverage is 70.21%.

@@            Coverage Diff             @@
##             main     #306      +/-   ##
==========================================
- Coverage   96.09%   95.50%   -0.59%     
==========================================
  Files          48       49       +1     
  Lines        2124     2181      +57     
==========================================
+ Hits         2041     2083      +42     
- Misses         83       98      +15     

Flag	Coverage Δ
unittests	95.50% <70.21%> (-0.59%)	⬇️

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

Impacted Files	Coverage Δ
doctr/file_utils.py	68.88% <68.88%> (ø)
doctr/__init__.py	100.00% <100.00%> (ø)
...tr/models/detection/differentiable_binarization.py	93.25% <0.00%> (-0.40%)	⬇️
doctr/datasets/ocr.py	91.89% <0.00%> (-0.22%)	⬇️
doctr/datasets/cord.py	97.50% <0.00%> (-0.07%)	⬇️
doctr/datasets/funsd.py	96.87% <0.00%> (ø)
doctr/datasets/sroie.py	94.73% <0.00%> (ø)
doctr/datasets/detection.py	97.14% <0.00%> (ø)
doctr/datasets/recognition.py	90.00% <0.00%> (ø)
... and 2 more

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[charlesmindee]

Thanks!