Add CHANGELOG.md files for Microsoft.Extensions.DataIngestion* projects (#7072)

* Initial plan

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

Co-authored-by: adamsitnik <[email protected]>

* Remove internal types from CHANGELOGs (MarkdownParser and ElementsChunker)

Co-authored-by: adamsitnik <[email protected]>

* Simplify CHANGELOGs to just "Initial preview release" for 10.0.0-preview.1

Co-authored-by: adamsitnik <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: adamsitnik <[email protected]>

Author: Copilot

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

@@ -0,0 +1,5 @@
+# Release History
+
+## 10.0.0-preview.1
+
+- Initial preview release

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

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+# Release History
+
+## 10.0.0-preview.1
+
+- Initial preview release

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

@@ -0,0 +1,5 @@
+# Release History
+
+## 10.0.0-preview.1
+
+- Initial preview release

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

@@ -0,0 +1,9 @@
+# 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