feat: add pdf support

[StevanFreeborn]

closes #18

• feat: add content type
• feat: add model for DocumentContent and DocumentSource
• tests: add end to end test using pdf support
• docs: add info about pdf support to README.md
• tests: add a test for converting DocumentContent

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.45%. Comparing base (24bd877) to head (dc9755c).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
- Coverage   98.66%   98.45%   -0.21%     
==========================================
  Files          64       66       +2     
  Lines        1200     1232      +32     
  Branches      101      101              
==========================================
+ Hits         1184     1213      +29     
  Misses          5        5              
- Partials       11       14       +3     

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