refactor: Removed models.export and added instructions in the documentation

[fg-mindee]

This PR introduces the following modifications:

• removes the doctr.models.export submodule
• removes its unittests
• added corresponding instructions about TFLite in the documentation

Any feedback is welcome!

[codecov]

Codecov Report

Merging #463 (7b483a4) into main (148ac00) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #463      +/-   ##
==========================================
- Coverage   95.79%   95.79%   -0.01%     
==========================================
  Files          96       95       -1     
  Lines        4023     3993      -30     
==========================================
- Hits         3854     3825      -29     
+ Misses        169      168       -1     

Flag	Coverage Δ
unittests	95.79% <ø> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
doctr/models/__init__.py	100.00% <ø> (ø)
...dels/detection/differentiable_binarization/base.py	91.97% <0.00%> (+0.61%)	⬆️

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[charlesmindee]

Thanks!