feat: Improved font resolution and page synthesis

[fg-mindee]

This PR introduces the following modifications:

• refactored all font selection code into a doctr.utils.fonts module (for character generation & page synthesis)
• fixed typo synthetize_page --> synthesize_page
• improve page synthesis thanks to the new font module
• added a synthesize method to Document and Page
• added synthesis visualization sample in the README
• updated unittests
• updated documentation

As shown in the README, here is how it renders:

Any feedback is welcome!

[charlesmindee]

Thanks!

[codecov]

Codecov Report

Merging #472 (52814d1) into main (342239a) will increase coverage by 0.01%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##             main     #472      +/-   ##
==========================================
+ Coverage   95.36%   95.37%   +0.01%     
==========================================
  Files          96       97       +1     
  Lines        4053     4065      +12     
==========================================
+ Hits         3865     3877      +12     
  Misses        188      188              

Flag	Coverage Δ
unittests	95.37% <96.42%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
doctr/utils/fonts.py	92.85% <92.85%> (ø)
doctr/datasets/classification/base.py	93.93% <100.00%> (+1.83%)	⬆️
doctr/io/elements.py	90.44% <100.00%> (+0.28%)	⬆️
doctr/models/recognition/__init__.py	100.00% <100.00%> (ø)
doctr/utils/visualization.py	93.57% <100.00%> (-0.06%)	⬇️

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