[Feat] Add reading order module

[felixdittrich92]

This PR:

• Add reading order module and cor tests

[codecov]

Codecov Report

❌ Patch coverage is 98.17352% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.69%. Comparing base (369f0aa) to head (e9b5b6d).

Files with missing lines	Patch %	Lines
doctr/models/reading_order/base.py	98.16%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2110      +/-   ##
==========================================
+ Coverage   96.65%   96.69%   +0.03%     
==========================================
  Files         166      168       +2     
  Lines        8707     8926     +219     
==========================================
+ Hits         8416     8631     +215     
- Misses        291      295       +4     

Flag	Coverage Δ
unittests	96.69% <98.17%> (+0.03%)	⬆️

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