[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.

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[bvolpato]

LGTM. Small non-blocking wording note inline.

[bvolpato]

non-blocking: could call these three components rather than three separate processes? The standalone cluster already has separate JobManager and TaskManager processes, while LOOPBACK hosts user code in the submitting Python process by default.