feat: Add page markers option to markdown export

[Adolar13]

This PR adds a new 'add_page_markers' option to the export_to_markdown and
save_as_markdown functions in DoclingDocument. When enabled, this option adds
page markers (e.g., ##PAGE 1##) to the markdown output, making it easier to
identify content from different pages in the exported document.

The implementation includes:

1. Adding the add_page_markers parameter to DocSerializer
2. Modifying the MarkdownDocSerializer.serialize method to add page markers
3. Adding tests to verify the functionality

This feature is useful for documents with multiple pages where maintaining
page boundaries in the exported markdown is important.

Fixes docling-project/docling#309 (comment)

Signed-off-by: Paul [email protected]

[mergify]

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