[Docs] Add a contributor guide for running Python on a local Flink cluster

[Eliaaazzz]

Adds contributor-docs/local-flink-python.md, a guide for running Beam Python pipelines against a local Flink standalone cluster: obtaining a Flink distribution, cluster configuration and startup, running a pipeline with FlinkRunner and --environment_type=LOOPBACK, troubleshooting, and teardown. Linked from the contributor docs README.

This is useful when embedded Flink is not enough, for example when validating unbounded source behaviour, checkpoint boundaries, or runner-visible job state in the Flink dashboard, since embedded mode starts a cluster only for the lifetime of one job.

Split out of #39212, which now carries only the Flink version reference updates on the runner page, so the two can be reviewed independently.

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Choose reviewer(s) and mention them in a comment (R: @username).
• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[github-actions]

Assigning reviewers:

R: @Abacn added as fallback since no labels match configuration

Note: If you would like to opt out of this review, comment assign to next reviewer.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).