docs: Added README specific for the API with route examples

[fg-mindee]

Following on some QA feedback, this PR adds a dedicated README for the API template. It provides curated examples of inputs and outputs on each route.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #612 (a3cbd30) into main (a792a82) will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #612      +/-   ##
==========================================
+ Coverage   96.03%   96.18%   +0.14%     
==========================================
  Files         110      111       +1     
  Lines        4265     4296      +31     
==========================================
+ Hits         4096     4132      +36     
+ Misses        169      164       -5     

Flag	Coverage Δ
unittests	96.18% <ø> (+0.14%)	⬆️

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

Impacted Files	Coverage Δ
doctr/datasets/datasets/base.py	95.45% <0.00%> (-0.11%)	⬇️
doctr/datasets/__init__.py	100.00% <0.00%> (ø)
doctr/datasets/iiit5k.py	96.77% <0.00%> (ø)
...dels/detection/differentiable_binarization/base.py	91.82% <0.00%> (+0.62%)	⬆️
doctr/utils/data.py	98.11% <0.00%> (+9.43%)	⬆️

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[charlesmindee]

Thanks for the PR !