Markdown header levels are all the same

### Bug

This is probably not a real bug but a feature request:

All headers in the generated markdown files are at the same level, i.e. ## .

For example, in the sample PDF to markdown conversion, section 3 has multiple sub-sections:

## 3 Processing pipeline
....
## 3.1 PDF backends
...
## 3.2 AI models

The correct header levels really should be something like this:

## 3 Processing pipeline
....
### 3.1 PDF backends
...
### 3.2 AI models
...

It would be great if the header level can be fixed, because this can keep the hierarchical structure of the original article.

### Steps to reproduce

Just run the sample code of this repo.
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### Docling version

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### Python version

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