[DOC] Add copy ingest processor

[gaobinlong]

What do you want to do?

• 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 the incoming release 2.12.0, we'll add a new ingest processor called copy processor, we should document the usage.

Please assign this issue to me, I'll open a PR for that.

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

opensearch-project/OpenSearch#11870

[vagimeli]

@gaobinlong @hdhalter I can support you on this. Please assign me to the PR once it's created.