refactor: restructure PDF/Image example document organization (#3410)

This PR aims to improve the organization and readability of our example
documents used in unit tests, specifically focusing on PDF and image
files.

### Summary
- Created two new subdirectories in the `example-docs` folder:
  - `pdf/`: for all PDF example files
  - `img/`: for all image example files
- Moved relevant PDF files from `example-docs/` to `example-docs/pdf/`
- Moved relevant image files from `example-docs/` to `example-docs/img/`
- Updated file paths in affected unit & ingest tests to reflect the new
directory structure

### Testing
All unit & ingest tests should be updated and verified to work with the
new file structure.

## Notes
Other file types (e.g., office documents, HTML files) remain in the root
of `example-docs/` for now.

## Next Steps
Consider similar reorganization for other file types if this structure
proves to be beneficial.

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Co-authored-by: ryannikolaidis <[email protected]>
Co-authored-by: christinestraub <[email protected]>

Author: christinestraub