[DOC][Breaking Change] Data Prepper: rename pipeline_configurations to extension in pipeline.yml

[chenqi0805]

What do you want to do?

• [ x ] Request a change to existing documentation
• [ ] Add new documentation
• [ ] Report a technical problem with the documentation
• [ ] Other

Tell us about your request. Provide a summary of the request and all versions that are affected. pipeline_configurations appearing in pipelines.yml causes confusion with pipeline definition. We will deprecate that keyword and rename it into extension.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.