[BP-2.0][FLINK-37162][docs] Add sinks.md to describe Flink's Data Sink API

[lvyanquan]

This closes #26013.

(cherry picked from commit 25f0cf3)

What is the purpose of the change

Backport of FLINK-37162.

Brief change log

Verifying this change

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 / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• e446719 Azure: FAILURE

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