[docu]: add documentation for datasets

[felixdittrich92]

This PR:

• add section in documentation how to use datasets
• add some dataset informations

Issue:
#855

Any feedback is welcome 🤗

[codecov]

Codecov Report

Merging #905 (94b732b) into main (3d645ce) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #905   +/-   ##
=======================================
  Coverage   94.73%   94.73%           
=======================================
  Files         134      134           
  Lines        5472     5472           
=======================================
  Hits         5184     5184           
  Misses        288      288           

Flag	Coverage Δ
unittests	94.73% <ø> (ø)

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

Impacted Files	Coverage Δ
doctr/datasets/generator/pytorch.py	100.00% <ø> (ø)
doctr/datasets/generator/tensorflow.py	100.00% <ø> (ø)
doctr/datasets/loader.py	100.00% <ø> (ø)
doctr/datasets/ocr.py	92.30% <ø> (ø)
doctr/transforms/modules/base.py	94.59% <0.00%> (ø)

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[felixdittrich92]

@fharper please review :D

I have attached the docu for you 👍
docu.zip

[fharper]

documentation looks good!

@charlesmindee can you review the small changes in the code please :)

[charlesmindee]

Thanks!