Document Python BigQuery CDC record format

[mukeshbhandarkar]

What does this PR do?

Documents the Python BigQuery Change Data Capture (CDC) input format for WriteToBigQuery.

The new documentation explains:

• the required STORAGE_WRITE_API, use_at_least_once, and use_cdc_writes settings
• primary key requirements
• the expected row_mutation_info / record PCollection structure
• UPSERT and DELETE mutation types
• change_sequence_number
• how Beam maps mutation metadata to BigQuery _CHANGE_TYPE and _CHANGE_SEQUENCE_NUMBER
• dictionary input requirements

The use_cdc_writes Python API documentation is also clarified.

Fixes #39768

Testing

• git diff --check
• Python 3.12 compilation
• Verified Hugo shortcode delimiters
• Confirmed only the intended documentation/API docstring files changed

I did not run ./gradlew :website:testWebsite locally because it requires building the website environment and related dependencies that are not available in my local setup.

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