[doc] Add Flink CDC integration content to Fluss documentation

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Description

Help users know they can use Flink CDC's rich connectors to bring data into Fluss. Flink CDC official docs reference: https://nightlies.apache.org/flink/flink-cdc-docs-release-3.5/docs/connectors/pipeline-connectors/fluss/ For now, we could adding Flink CDC integration content under the "Flink Engine" section of the documentation. We could split the introduction into two parts:

• Flink CDC Pipeline Connector: Define the synchronization pipeline for PostgreSQL data to Fluss by writing a pipeline YAML file.
• Flink CDC SQL Connector: Sync PostgreSQL data to Fluss with simple SQL statements.

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Willingness to contribute

• I'm willing to submit a PR!