[docs] update export page to ONNX

[felixdittrich92]

This PR:

• Updates export page in docs
• Replaces outdated content which does not work (Onnx works for both TF and PT - we decided to support Onnx as format for model export)
• Adds samples for FP16 TF / PT

Any feedback is welcome 🤗

[codecov]

Codecov Report

Merging #1261 (5f0e89c) into main (39427d5) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1261      +/-   ##
==========================================
- Coverage   95.75%   95.73%   -0.02%     
==========================================
  Files         154      154              
  Lines        6901     6901              
==========================================
- Hits         6608     6607       -1     
- Misses        293      294       +1     

Flag	Coverage Δ
unittests	95.73% <ø> (-0.02%)	⬇️

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see 1 file with indirect coverage changes