feat: Add remaining API methods and simplify shape example

- Add document metadata APIs:
  - set_document_title()
  - get_document_statistics()
  - update_document_statistics()
- Add page layout APIs:
  - set_custom_page_size_mm()
  - set_page_margins_inches()
- Export DocumentStatistics type
- Simplify shape_document.rs example (full shape API planned for v0.4.0)
- Fix text_box_document.rs metadata chaining
- Fix page_layout_test.rs Option unwrapping

Library compiles successfully. Some tests still need adjustments.

Author: claude