refactor: migrate ai.forecast from AIAccessor to bbq

[sycai]

• The original API is kept for backward compatibility
• A warning will be thrown if the user tries to use df.ai accessor
• Updated the AI forecast notebook to showcase the new usage
• Removed the AI accessor doc from our public API doc page

I also updated the parameters to make them consistent with the SQL version:

• renamed "data_column" -> "data_col", "timestamp_column" -> "timestamp_col" etc. to align with the SQL function naming scheme
• made "data_col" and "timestamp_col" keyword-only params, which is consistent with the SQL function definition.
• added "context_window", which was introduced later but not present in the aiaccessor forecast

SQL func doc reference: https://docs.cloud.google.com/bigquery/docs/reference/standard-sql/bigqueryml-syntax-ai-forecast

Fixes #454138453 🦕

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