Add SPLIT_PAGE_CONTENT_ONLY output mode to HTML export

` blocks, each with a `data-page` attribute for page numbering, but *without* images or table markup.

**Proposed solution:**
- Add a new output style (`SPLIT_PAGE_CONTENT_ONLY`) to the `HTMLOutputStyle` enum.
- In the `HTMLDocSerializer.serialize_doc` method, implement a block that:
- Splits the document per page (like current split logic),
- Outputs each page as `

` (where `N` is the page number),
- Does *not* include images or table structure.

This would allow users to directly export clean, page-wise HTML content for further web embedding or processing.

**Suggested code changes:**
- Extend the `HTMLOutputStyle` enum.
- Add corresponding logic in the serializer as outlined above.

Thank you!