[docs] Add note about labeling tool to training section

[felixdittrich92]

This PR:

• small documentation addition to https://github.com/text2knowledge/docTR-Labeler

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.56%. Comparing base (b0d2728) to head (3502d61).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1839      +/-   ##
==========================================
- Coverage   96.58%   96.56%   -0.03%     
==========================================
  Files         165      165              
  Lines        7940     7940              
==========================================
- Hits         7669     7667       -2     
- Misses        271      273       +2     

Flag	Coverage Δ
unittests	96.56% <ø> (-0.03%)	⬇️

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[odulcy-mindee]

Ty, really great addition !