feat: Added PDF reading from binary stream

[fg-mindee]

This PR introduces the following modifications:

• added the possibility to read a PDF from a byte stream
• added corresponding unittest

Any feedback is welcome!

[codecov]

Codecov Report

Merging #83 (e8ea9eb) into main (e9bfd3e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #83   +/-   ##
=======================================
  Coverage   96.71%   96.72%           
=======================================
  Files          24       24           
  Lines         791      793    +2     
=======================================
+ Hits          765      767    +2     
  Misses         26       26           

Flag	Coverage Δ
unittests	96.72% <100.00%> (+<0.01%)	⬆️

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

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

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e9bfd3e...d0bcb59. Read the comment docs.