[builder] Add objectness scores

[felixdittrich92]

This PR:

• unify detection predictor output (N, 5) / (N, 5, 2) --> last is objectness_score
• add objectness score to Document output
• update API template
• update docs
• add corresponding tests

Use case: enables the end user to filter out some detected artefacts for example

Any feedback is welcome 🤗

NOTE: API tests will fail until merged :)

Related to: #1612

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.39%. Comparing base (5eda559) to head (52a6850).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1625   +/-   ##
=======================================
  Coverage   96.39%   96.39%           
=======================================
  Files         164      164           
  Lines        7716     7744   +28     
=======================================
+ Hits         7438     7465   +27     
- Misses        278      279    +1     

Flag	Coverage Δ
unittests	96.39% <100.00%> (+<0.01%)	⬆️

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[felixdittrich92]

ref.: #1612