feat: Added image reading utilities

[fg-mindee]

Following up on #1, this PR introduces the following modifications:

• implements an image reading function
• added corresponding unittest
• added it to the documentation

Successfully merging this PR will close #1

Any feedback is welcome!

[codecov]

Codecov Report

Merging #30 (f046a2e) into main (7a290a4) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   98.85%   98.89%   +0.04%     
==========================================
  Files           9        9              
  Lines         174      181       +7     
==========================================
+ Hits          172      179       +7     
  Misses          2        2              

Impacted Files	Coverage Δ
doctr/documents/reader.py	100.00% <100.00%> (ø)

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