Improve synthesize_page and font fallback

[felixdittrich92]

This PR:

• Improve page recunstruction
• Improve font fallback
• Add cor tests

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.71%. Comparing base (5516d31) to head (3c750bb).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
doctr/utils/reconstitution.py	93.47%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2098      +/-   ##
==========================================
+ Coverage   96.70%   96.71%   +0.01%     
==========================================
  Files         166      166              
  Lines        8582     8652      +70     
==========================================
+ Hits         8299     8368      +69     
- Misses        283      284       +1     

Flag	Coverage Δ
unittests	96.71% <94.59%> (+0.01%)	⬆️

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