feat: Added page selection & JSON display to demo

[fg-mindee]

This PR introduces the following modifications:

• added tip to enlarge images
• switched to 4 columns display
• added page selection (for multi-page PDF)
• speeded up analysis by only analyzing the selected page
• added JSON output display (foldable, and can be copied to clipboard)
• updated demo display in README

Here is how it renders:

Closes #234

Any feedback is welcome!

[codecov]

Codecov Report

Merging #369 (a46c527) into main (c18025b) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #369      +/-   ##
==========================================
- Coverage   96.18%   96.15%   -0.03%     
==========================================
  Files          83       83              
  Lines        3461     3461              
==========================================
- Hits         3329     3328       -1     
- Misses        132      133       +1     

Flag	Coverage Δ
unittests	96.15% <ø> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
...dels/detection/differentiable_binarization/base.py	91.30% <0.00%> (-0.63%)	⬇️

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[charlesmindee]

Thanks for the tuning