Examples - Provide additional sample code for common operations

[yveszoundi]

Rationale

I felt like more examples would have helped me transition quicker from other libraries (poppler-rs + cairo-rs and lopdf).

Proposal

I can send a pull request with basic examples demonstrating the following:

• Extract PDF pages into images
• Concatenate/Merge PDF documents
• Convert images to PDF documents (with and without OCR, for OCR I would need to send another PR as it's not exposed in the high-level API)