initial autocdc type1 docs

[suhuruli]

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What changes were proposed in this pull request?

This PR documents Auto CDC (Change Data Capture) support for Spark Declarative Pipelines (SDP) in the programming guide.

It adds a new section, "Change Data Capture (CDC) with Auto CDC," to docs/declarative-pipelines-programming-guide.md, placed after the SQL programming section and before "Writing Data to External Targets with Sinks." The section covers:

• Concept - what Auto CDC does, its SCD Type 1 semantics (keep only the current version of each row), and how it handles inserts, updates, deletes, and out-of-order/duplicate events, with a worked before/after example.
• Requirements - the target must be a pre-created streaming table, the source must be a streaming source, and keys plus a sequencing expression are required.
• Python API reference - the create_auto_cdc_flow function with a full parameter table.
• SQL reference - both supported syntax forms (CREATE FLOW ... AS AUTO CDC INTO and CREATE STREAMING TABLE ... FLOW AUTO CDC), including required clause ordering.
• End-to-end quickstart - a spark-pipelines init -> run walkthrough.
• How-tos - handling deletes, selecting columns, out-of-order events, composite keys, and changing the key set (full refresh).
• Limitations/considerations - SCD Type 1 only, streaming-table target only, immutable key set, and table-format requirements.

No functional code is changed; this is documentation only.

Why are the changes needed?

AutoCDC Type 1 recently merged into 4.2, this adds prog guide.

Does this PR introduce any user-facing change?

Yes, but documentation only. It adds a new section to the Spark Declarative Pipelines programming guide. There is no change to behavior, APIs, or output.

How was this patch tested?

No tests were added; this is a documentation-only change. The Markdown was verified locally to ensure it is ASCII-only (per the repository convention) and that the internal anchor link (#spark-pipelines-run) resolves to an existing heading. All code snippets and API parameters were cross-checked against the current implementation (create_auto_cdc_flow, the autoCdcCommand SQL grammar, and the pipelines.proto definitions).

Was this patch authored or co-authored using generative AI tooling?
Yes