[FLINK-23140][docs] Fix SourceFunction documentation link

[Vamsi-klu]

What is the purpose of the change

This PR fixes the DataStream Data Sources documentation for the legacy source function APIs.

The current documentation mentions SourceFunction, ParallelSourceFunction, and RichParallelSourceFunction together when explaining how users can implement custom sources with StreamExecutionEnvironment.addSource(...). SourceFunction was updated to link to the current legacy source package, but the two related APIs in the same sentence were still left as plain code text.

To keep the documentation consistent and easier to navigate, this PR links all three legacy source APIs to their current source files:

• SourceFunction
• ParallelSourceFunction
• RichParallelSourceFunction

The same update is applied to both the English and Chinese DataStream documentation.

This closes FLINK-23140.

Brief change log

• Updated the English DataStream Data Sources documentation to link all three legacy source function APIs.
• Updated the Chinese DataStream Data Sources documentation with the same set of legacy source API links.
• Kept the change limited to documentation; no runtime code, API behavior, or dependency changes are included.

Verifying this change

This change is a documentation-only update without new test coverage.

Verified locally with:

• test -f flink-runtime/src/main/java/org/apache/flink/streaming/api/functions/source/legacy/SourceFunction.java
• test -f flink-runtime/src/main/java/org/apache/flink/streaming/api/functions/source/legacy/ParallelSourceFunction.java
• test -f flink-runtime/src/main/java/org/apache/flink/streaming/api/functions/source/legacy/RichParallelSourceFunction.java
• git diff --check

The verification confirms that each linked target file exists and that the Markdown diff does not contain whitespace errors.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

---

Was generative AI tooling used to co-author this PR?

• Yes (OpenAI Codex)

Generated-by: OpenAI Codex

[flinkbot]

CI report:

• f0e6206 Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[spuru9]

Can you update the description as needed by the PULL_REQUEST template as well.

[Vamsi-klu]

Thanks, updated.

I pushed the follow-up docs change so SourceFunction, ParallelSourceFunction, and RichParallelSourceFunction are linked consistently in both the English and Chinese DataStream docs. I also updated the PR description to include the remaining PULL_REQUEST_TEMPLATE.md sections, including the impact checklist, documentation section, verification details, and AI tooling disclosure.

[spuru9]

LGTM