docs: Restructured documentation

[fg-mindee]

This PR introduces the following modifications:

• removes the mention that PyTorch is in beta in the README
• in the documentation, split benchmark & export model from actual docstrings: "Using DocTR" section doesn't include any function/class documentation but explains how some features work and provides benchmarks. Hyperlinks were added so that users can easily access the signature.

Here is the new navigation menu:

Please note that this doesn't include the PyTorch benchmark for now!

Any feedback is welcome!

[codecov]

Codecov Report

Merging #519 (91ff01c) into main (fa26b7a) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #519      +/-   ##
==========================================
- Coverage   95.40%   95.36%   -0.05%     
==========================================
  Files         109      109              
  Lines        4183     4183              
==========================================
- Hits         3991     3989       -2     
- Misses        192      194       +2     

Flag	Coverage Δ
unittests	95.36% <ø> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
doctr/models/detection/zoo.py	100.00% <ø> (ø)
doctr/models/recognition/master/tensorflow.py	96.47% <ø> (ø)
doctr/models/recognition/zoo.py	100.00% <ø> (ø)
doctr/models/zoo.py	100.00% <ø> (ø)
doctr/models/builder.py	92.70% <0.00%> (-1.05%)	⬇️
...dels/detection/differentiable_binarization/base.py	91.19% <0.00%> (-0.63%)	⬇️

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[charlesmindee]

Thanks!