docs: add per-module README files with codecov badges

Create individual README files for each production module:
- docling-core/README.md
- docling-serve/docling-serve-api/README.md
- docling-serve/docling-serve-client/README.md

Each README includes:
- Module-specific Codecov badge with flag parameter
- Link to detailed coverage reports on Codecov

This provides granular coverage visibility as requested by @edeandrea in PR #406

Co-authored-by: Bob <bob@codeium.com>
Signed-off-by: Rohan <rohan.kurian@ibm.com>

Author: Rohan

docling-core/README.md

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+# Docling Core
+
+[![codecov](https://codecov.io/gh/docling-project/docling-java/branch/main/graph/badge.svg?flag=docling-core)](https://codecov.io/gh/docling-project/docling-java/tree/main/docling-core)
+
+Core module for Docling Java.
+
+## Coverage
+
+View detailed coverage reports for this module on [Codecov](https://codecov.io/gh/docling-project/docling-java/tree/main/docling-core).

docling-serve/docling-serve-api/README.md

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+# Docling Serve API
+
+[![codecov](https://codecov.io/gh/docling-project/docling-java/branch/main/graph/badge.svg?flag=docling-serve-api)](https://codecov.io/gh/docling-project/docling-java/tree/main/docling-serve/docling-serve-api)
+
+API module for Docling Serve.
+
+## Coverage
+
+View detailed coverage reports for this module on [Codecov](https://codecov.io/gh/docling-project/docling-java/tree/main/docling-serve/docling-serve-api).

docling-serve/docling-serve-client/README.md

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+# Docling Serve Client
+
+[![codecov](https://codecov.io/gh/docling-project/docling-java/branch/main/graph/badge.svg?flag=docling-serve-client)](https://codecov.io/gh/docling-project/docling-java/tree/main/docling-serve/docling-serve-client)
+
+Client module for Docling Serve.
+
+## Coverage
+
+View detailed coverage reports for this module on [Codecov](https://codecov.io/gh/docling-project/docling-java/tree/main/docling-serve/docling-serve-client).