feat: add chart serialization support for pptx files and related classes

[valentin-best]

This PR introduces support for chart items in the document serialization and manipulation framework. The changes add a new ChartItem type, its associated data model, and integrate chart serialization into the existing system for Markdown, HTML, and DocTags formats. Additionally, methods for adding and inserting charts into documents are provided, along with updates to document structure and indexing to accommodate charts.

Chart Item Support

• Added the ChartItem class and its data model ChartData, enabling charts to be represented as items within documents. Includes methods for exporting charts to Markdown and Pandas DataFrame. [1] [2]
• Updated the document structure to include a charts list, and extended document indexing and update logic to handle chart items. [1] [2] [3]

Chart Manipulation Methods

• Added add_chart and insert_chart methods to DoclingDocument for creating and inserting chart items, similar to how tables and pictures are handled. [1] [2]
• Updated internal item appending logic to support chart items.

Chart Serialization Integration

• Introduced BaseChartSerializer abstract class and implemented format-specific serializers: MarkdownChartSerializer, HTMLChartSerializer, and DocTagsChartSerializer. These serializers are integrated into the respective document serializers. [1] [2] [3] [4]
• Registered chart serializers in the document serializer classes for Markdown, HTML, and DocTags, enabling serialization of chart items alongside other document elements. [1] [2] [3]

Type and Import Updates

• Updated imports and type definitions throughout the codebase to recognize ChartItem and ChartData, ensuring correct type checking and serialization. [1] [2] [3] [4] [5] [6]

Serialization Logic Extension

• Extended the main serialization logic to handle ChartItem instances, delegating to the appropriate chart serializer based on document format.

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