Add support for html_split_page format

When I use `to_formats: "html_split_page"`, I get response with all fields being null:
```
"document": {
"filename": null,
"md_content": null,
"json_content": null,
"html_content": null,
"text_content": null,
"doctags_content": null
},
```

If I pass `to_formats: "html"`, I get response with the proper value in `document.html_content`.
I've tried to parse the same file with docling CLI, without docling-serve, and it works just fine with `html_split_page` format.

Also, `html_split_page` is not available in the UI, but it's being listed in api docs page.

Reproduce the docling-serve API request with to_formats set to "html_split_page" and compare it with the working "html" response and the standalone Docling CLI. Trace the API format handling and the UI format selector; done means the split-page response is populated and the format is available consistently in the UI and API.

Written by the indexing model from the issue text.