[yaml] add About Beam YAML guide

[Polber]

What would you like to happen?

What would you like to happen?
This bug is part of the Beam YAML milestone. More information about Beam YAML can be found here: https://github.com/apache/beam/tree/master/sdks/python/apache_beam/yaml#beam-yaml-api

A Beam YAML contribution guide can be found here: https://s.apache.org/beam-yaml-contribute

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Beam YAML should have an About Guide that describes the feature in more detail than the README.md. This doc should ideally describe all supported transforms and how to define them in a YAML pipeline. It should also include the concepts in the README's including syntax, mapping, error handling, running the pipeline and limitations of Beam YAML.

Issue Priority

Priority: 2 (default / most feature requests 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: Spark Runner
• Component: Flink Runner
• Component: Samza Runner
• Component: Twister2 Runner
• Component: Hazelcast Jet Runner
• Component: Google Cloud Dataflow Runner

[smeet07]

.take-issue

[smeet07]

@AnandInguva could you give a general sense of direction to solve this issue?

[AnandInguva]

@smeet07 I think @Polber is the right POC on this issue. @Polber Can you help @smeet07 on #28696 (comment)?