[doc] Add Flink CDC integration content to Fluss documentation

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### 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.

Feel free to ask for help if you have any problems with this issue.

### Willingness to contribute

- [ ] I'm willing to submit a PR!