[misc] Change prefered backend from tf to torch

[felixdittrich92]

This PR:

• Change docTR's "prefered" backend from TF to PyTorch if both frameworks are installed
• Change the API template backend to PyTorch
• Documentation is build from code base with PyTorch as prefered backend now
• Some documentation fixes

Any feedback is welcome 🤗

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.62%. Comparing base (83f1bc5) to head (9a619ce).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1779      +/-   ##
==========================================
+ Coverage   96.56%   96.62%   +0.05%     
==========================================
  Files         164      165       +1     
  Lines        7895     7901       +6     
==========================================
+ Hits         7624     7634      +10     
+ Misses        271      267       -4     

Flag	Coverage Δ
unittests	96.62% <100.00%> (+0.05%)	⬆️

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