docs: Record DynamoDB persistent store support in SDK metadata and fix stale library docs

[beekld]

Records the C++ server SDK's DynamoDB persistent store support in SDK metadata and corrects the DynamoDB source library's own docs, which still described the library as unfinished.

• Adds storingDataDynamodb (introduced 3.13) to the cpp-server-sdk entry in .sdk_metadata.json, so DynamoDB shows up as a supported persistent store in the SDK feature matrix. This is the LazyLoad/persistent-store capability (the DynamoDB analog of storingDataRedis).
• libs/server-sdk-dynamodb-source/README.md: updated to reflect that the Big Segments store has launched.
• libs/server-sdk-dynamodb-source/docs/doc.md: replaces the "currently a scaffold" text with Doxygen links to the shipped DynamoDBDataSource and DynamoDBBigSegmentStore.

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Note

Overview
Records DynamoDB persistent-store support for the C++ server SDK and updates the DynamoDB source library docs that still described it as unfinished.

Adds storingDataDynamodb (introduced in 3.13) to .sdk_metadata.json so DynamoDB appears in the SDK feature matrix alongside Redis.

Updates the DynamoDB source README and Doxygen overview to remove the "scaffold / coming later" language and point to the shipped DynamoDBDataSource and DynamoDBBigSegmentStore APIs.

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