[Feature Request] Add README in example system pipeline plugin for system ingest pipeline extension points

[q-andy]

Is your feature request related to a problem? Please describe

System ingest pipelines added in #17817 are a new pipeline type which allow plugins to dynamically inject and execute processors based on the index mapping or other conditional checks. In essence, system pipelines can allow plugin developers to implement features that completely abstract pipelines/processor executions from the user.

Currently, system ingest pipelines are used to support semantic field (RFC), where we chunk and generate embeddings for semantic field text without the user having to manually configure pipelines themselves.

While system pipelines were developed with semantic field in mind, the extension points are flexible and may be used to develop future field types implemented by other plugins as well.

Describe the solution you'd like

We should document in detail nuances of how system pipelines work outside of the code, such as

• Interactions with bulk update (see also [Feature Request] Support system generated ingest pipelines for bulk update operations #18276)
• Order of pipeline execution
• System pipeline caching
• Configurability options (enable/disable cluster setting)

Since this documentation is targeted at developers, I believe this should go in a readme in the code base itself, rather than in the documentation website which is for users of OpenSearch

Related component

Indexing

Describe alternatives you've considered

not having explicit documentation, letting future developers read the code

Additional context

No response

[owaiskazi19]

@q-andy, suggest moving the documentation for this feature to our main documentation instead of keeping it as a README in the codebase. This would align with our existing documentation pattern for new features and make it easier for users to find.