Add CHANGELOG.md files for Microsoft.Extensions.DataIngestion* projects

src/Libraries/Microsoft.Extensions.DataIngestion.Abstractions/CHANGELOG.md

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+# Release History
+
+## 10.0.0-preview.1
+
+- Initial preview release of Microsoft.Extensions.DataIngestion.Abstractions
+- Introduced `IngestionDocument` class for representing format-agnostic document containers
+- Introduced `IngestionDocumentElement` abstract base class for document elements
+- Introduced document element types:
+  - `IngestionDocumentSection` - Represents a section or page in a document
+  - `IngestionDocumentParagraph` - Represents a paragraph
+  - `IngestionDocumentHeader` - Represents a header with optional level
+  - `IngestionDocumentFooter` - Represents a footer
+  - `IngestionDocumentTable` - Represents a table with 2D cell array
+  - `IngestionDocumentImage` - Represents an image with optional binary content and alternative text
+- Introduced `IngestionChunk<T>` class for representing content chunks
+- Introduced `IngestionChunker<T>` abstract base class for splitting documents into chunks
+- Introduced `IngestionDocumentReader` abstract base class for reading source content and converting to documents
+- Introduced `IngestionDocumentProcessor` abstract base class for processing documents
+- Introduced `IngestionChunkProcessor<T>` abstract base class for processing chunks
+- Introduced `IngestionChunkWriter<T>` abstract base class for writing chunks to storage

src/Libraries/Microsoft.Extensions.DataIngestion.MarkItDown/CHANGELOG.md

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+# Release History
+
+## 10.0.0-preview.1
+
+- Initial preview release of Microsoft.Extensions.DataIngestion.MarkItDown
+- Introduced `MarkItDownReader` class for converting documents to markdown using the MarkItDown CLI
+- Introduced `MarkItDownMcpReader` class for converting documents using MarkItDown Model Context Protocol (MCP) server

src/Libraries/Microsoft.Extensions.DataIngestion.Markdig/CHANGELOG.md

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+# Release History
+
+## 10.0.0-preview.1
+
+- Initial preview release of Microsoft.Extensions.DataIngestion.Markdig
+- Introduced `MarkdownReader` class for reading markdown documents and converting them to `IngestionDocument`

src/Libraries/Microsoft.Extensions.DataIngestion/CHANGELOG.md

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+# Release History
+
+## 10.1.0-preview.1
+
+- Introduced `SectionChunker` class for treating each document section as a separate entity (https://github.com/dotnet/extensions/pull/7015)
+
+## 10.0.0-preview.1
+
+- Initial preview release of Microsoft.Extensions.DataIngestion
+- Introduced `IngestionPipeline<T>` class for orchestrating document ingestion workflows
+- Introduced `IngestionPipelineOptions` class for configuring pipeline behavior
+- Introduced `IngestionResult` class for representing ingestion operation results
+- Introduced chunker implementations:
+  - `HeaderChunker` - Splits documents based on headers and their levels
+  - `SemanticSimilarityChunker` - Splits documents based on semantic similarity using embeddings
+- Introduced `IngestionChunkerOptions` class for configuring chunker behavior (token limits, overlap, etc.)
+- Introduced document processors/enrichers:
+  - `ClassificationEnricher` - Enriches document metadata with classifications
+  - `KeywordEnricher` - Enriches document metadata with keywords
+  - `SentimentEnricher` - Enriches document metadata with sentiment analysis
+  - `SummaryEnricher` - Enriches document metadata with summaries
+  - `ImageAlternativeTextEnricher` - Enriches images with alternative text descriptions
+- Introduced `EnricherOptions` class for configuring enricher behavior
+- Introduced `VectorStoreWriter<T>` class for writing chunks to vector stores
+- Introduced `VectorStoreWriterOptions<T>` class for configuring vector store writing behavior