[Task]: Document WaitOn / WaitOn transforms in Beam Transform catalog

[tvalentyn]

What needs to happen?

Transform catalog pages for Python and Java don't mention Wait.On transform, potentially making it harder to discover:

https://beam.apache.org/documentation/transforms/python/overview/
https://beam.apache.org/documentation/transforms/java/overview/

See:

beam/sdks/python/apache_beam/transforms/util.py

Line 1849 in b26ecf1

class WaitOn(PTransform):

beam/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/Wait.java

Line 79 in b26ecf1

public class Wait {

Let's document these transforms.

Issue Priority

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

Issue Components

• Component: Python SDK
• Component: Java SDK
• Component: Go SDK
• Component: Typescript SDK
• Component: IO connector
• Component: Beam YAML
• Component: Beam examples
• Component: Beam playground
• Component: Beam katas
• Component: Website
• Component: Infrastructure
• Component: Spark Runner
• Component: Flink Runner
• Component: Samza Runner
• Component: Twister2 Runner
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• Component: Google Cloud Dataflow Runner

[ashmod]

Raised #35647 , please review