Charts Extraction from PPTX files

### Requested feature

Add support for extracting charts from PPTX files in Docling.
Currently, Docling ignores charts when parsing .pptx presentations, which means valuable data (e.g., bar/line/pie charts) is lost in downstream processing.

The feature could expose chart metadata and data series in a structured format (e.g., JSON) so that downstream tools can either visualize them or analyze the data programmatically.

Example JSON output for a simple bar chart:
```jsonc
{
"slide": 3,
"shape_id": 12,
"type": "clusteredColumn",
"title": "Revenue by Region",
"categories": ["Q1","Q2","Q3","Q4"],
"series": [
{"name":"EMEA","values":[12.3,14.2,15.0,16.8]},
{"name":"AMER","values":[10.1,11.0,13.4,14.0]}
]
}
```
This would make Docling much more useful for users working with business presentations where charts are as important as text.

### Alternatives

Currently, the only workaround is to parse PPTX files manually using python-pptx, walk through all shapes, extract the chart data and build a custom JSON output.
While this works, it requires writing and maintaining additional code outside Docling, and prevents a unified API for accessing text + charts.

### Additional context
I’d be happy to contribute an initial implementation using python-pptx, at least for the most common chart types (bar/column/line/pie).
Would you accept a PR adding this feature behind a flag or as part of the default PPTX parsing pipeline?