[FLINK-40547][docs] Add page on interpreting Flink's changelog

[gustavodemorais]

What is the purpose of the change

Adds a docs page explaining how to correctly interpret a Flink changelog stream (RowKind, changelog modes, and the primary-key-or-not consumer algorithm), with worked examples for keyed and keyless streams, partitioning behavior, and common pitfalls. Links it from the Process Table Functions docs instead of duplicating the changelog-basics explanation there.

Brief change log

• Add docs/content/docs/concepts/sql-table-concepts/interpreting_changelog.md
• Cover RowKind, Append/Upsert/Retract changelog modes, and the primary-key-based consumer algorithm
• Add worked examples for keyed and keyless changelogs, including partition distribution and transient intermediate states
• Replace the "Change Data Capture Basics" section in ptfs.md with a link to the new page

Verifying this change

This change is a docs-only addition; no new tests apply.

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? (N/A - this PR is itself a documentation page: docs/content/docs/concepts/sql-table-concepts/interpreting_changelog.md)

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

• Yes (please specify the tool below)

2.1.259 (Claude Code) with Sonnet 5

[flinkbot]

CI report:

• e70060e Azure: SUCCESS

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