[Task]: Document `extra_transforms` block for `chain` type pipelines

[hjtran]

What needs to happen?

The extra_transforms block is a nice little feature of Beam YAML chain pipelines, but it seems to only be documented in the Apache Beam YAML Error Handling docs page. It seems worth mentioning under the chaining transforms docs section as well

Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

Issue Components

• Component: Python SDK
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[derrickaw]

.take-issue