feat: Added line reading for source PDFs

[fg-mindee]

This PR introduces the following modifications:

• adds get_lines methods on PDF objects to aggregate words into lines on source PDFs
• adds unittest
• adds entry in the documentation

Any feedback is welcome!

[charlesmindee]

Thanks!

[codecov]

Codecov Report

Merging #707 (a10548a) into main (cf97d9e) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #707      +/-   ##
==========================================
+ Coverage   96.12%   96.18%   +0.05%     
==========================================
  Files         124      124              
  Lines        4648     4668      +20     
==========================================
+ Hits         4468     4490      +22     
+ Misses        180      178       -2     

Flag	Coverage Δ
unittests	96.18% <100.00%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
doctr/io/pdf.py	100.00% <100.00%> (ø)
doctr/models/builder.py	96.49% <0.00%> (-2.64%)	⬇️
...dels/detection/differentiable_binarization/base.py	93.33% <0.00%> (+2.77%)	⬆️

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