[DOC] Pipeline ingest processor supports ignore_missing_pipeline parameter

[gaobinlong]

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• 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.

In 2.13.0, we'll introduce a new parameter ignore_missing_pipeline for the pipeline ingest processor, we need to document that.

PUT _ingest/pipeline/outer
{
  "description": "outer pipeline",
  "processors": [
    {
      "pipeline": {
        "name": "inner",
        "ignore_missing_pipeline": true
      }
    }
  ]
}

But now we don't have documentation for the pipeline ingest processor, we need to add that document firstly.

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

opensearch-project/OpenSearch#12476

[gaobinlong]

The related code PR hasn't been merged because the idea in that PR isn't accepted by the maintainers, so this change doesn't target for 2.13.0, now I'll close this issue and if the code PR is merged, I'll open a new doc issue for it.

[hdhalter]

Thanks, @gaobinlong !