[Layout] Add layout element & extend vis

[felixdittrich92]

This PR:

• Add LayoutElement for DocumentBuilder
• Extend visualization
• Update tests

NOTE: defusedxml replaced by xml.etree

[codecov]

Codecov Report

❌ Patch coverage is 97.91667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.89%. Comparing base (bd2dec8) to head (029150a).

Files with missing lines	Patch %	Lines
doctr/io/elements.py	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2074      +/-   ##
==========================================
- Coverage   96.91%   96.89%   -0.02%     
==========================================
  Files         156      156              
  Lines        7379     7415      +36     
==========================================
+ Hits         7151     7185      +34     
- Misses        228      230       +2     

Flag	Coverage Δ
unittests	96.89% <97.91%> (-0.02%)	⬇️

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