feat: support page numbers in Markdown page breaks

[gyx09212214-prog]

Summary

Adds named page-number placeholders for Markdown page_break_placeholder values.

Today MarkdownParams(page_break_placeholder="<!-- page {page_no} -->") emits {page_no} literally. The internal page-break marker already carries both previous and next page numbers, so this change formats the placeholder when the final Markdown text is assembled.

Supported placeholders:

• {page_no}: the page after the break, matching the common “page marker before page N” use case
• {next_page}: same value as {page_no}
• {prev_page}: the page before the break

Existing literal placeholders such as "<!-- page break -->", "", and None keep their current behavior.

This helps document/RAG pipelines preserve page-level provenance in Markdown exports, which is useful for citations and traceable retrieval chunks. It also addresses the page-number placeholder part of docling-project/docling#2036.

Tests

• python -m pytest test/test_serialization.py -q (48 passed)
• python -m py_compile docling_core/transforms/serializer/markdown.py test/test_serialization.py
• git diff --check
• python -m ruff check docling_core/transforms/serializer/markdown.py test/test_serialization.py

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