[Website] add drain update to docs

[stankiewicz]

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Summary of Changes

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This pull request updates the Apache Beam documentation to provide guidance on handling looping timers during pipeline drain operations. It introduces a new section in the programming guide and clarifies runner-specific support for this feature, ensuring users can better manage pipeline termination.

Highlights

• Documentation Update: Added a new section on looping timers to the programming guide, including code examples for Java, Python, and Go.
• Drain Capability Documentation: Updated the capability matrix and blog post to reflect the current status of looping timer support during pipeline drain.
• Changelog Entry: Added a note regarding the new indicator for aggregations and timers firing during a pipeline drain.

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Code Review

This pull request introduces documentation and examples for managing looping timers during pipeline drains, specifically highlighting the use of the CausedByDrain parameter in the Java SDK. It also updates the capability matrix and blog content to reflect Dataflow's support for this feature. Review feedback correctly identified a missing OutputReceiver parameter in the Java documentation example and a type mismatch in the Go example that would prevent the code from functioning as intended.

[github-actions]

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R: @shunping for label website.

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