[DOC] Move concurrent search plugin details to core repo

[jed326]

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. From https://github.com/opensearch-project/documentation-website/pull/8181#issuecomment-2341205852, we want to move the plugin implementation details to the developer guide in the core repo. However, there isn't really any existing search plugin developer information in the core repo today, so creating this issue to track this as we add more search plugin information to core.

*Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.

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