Add a field to DoclingDocument for metadata

[ceberam]

The DoclingDocument class defines a unified data model for rich document representation and the docling library provides tools for conversion of common document formats (e.g., PDF, MS Office formats, and HTML) into DoclingDocument objects.

Currently, DoclingDocument data model addresses the content of a document but common formats typically include structured information on top of that content (metadata), which is not currently represented in DoclingDocument.
Including such metadata would be beneficial for handling document collections and strengthening downstream applications like enterprise search and RAG on document collections.

Examples of metadata fields are:

• in PDF documents: title, author, subject, content creator, keywords, creation date, modification date
• in MS Office documents: title, author, subject, keywords, company, category, comments
• HTML: description, title, country code (and, in general, any content in <meta> tag).

This feature requests consists of:

• Adding a field in DoclingDocument for document metadata.
• Identifying and defining a set of fields for the metadata that is general enough for common document types.
• Eventually addressing the use case of custom metadata fields that are specific to a document type and that could be populated by docling's PDF parser or document backends.