docs: add Engine Refactoring report for v3.4.0

[tkykenmt]

Summary

This PR adds documentation for the Engine Refactoring feature in OpenSearch v3.4.0.

Changes in v3.4.0

• Moved prepareIndex and prepareDelete to Engine class - These methods are now instance methods on the Engine class instead of static methods on IndexShard, allowing custom engines to override document preparation logic.
• Made NoOpResult constructors public - Custom engine implementations can now create NoOpResult instances when overriding the noOp() method.

Reports Created

• Release report: docs/releases/v3.4.0/features/opensearch/engine-refactoring.md
• Feature report: docs/features/opensearch/engine-api.md (updated)

Related PRs

• #19551 - Refactor to move prepareIndex and prepareDelete methods to Engine class
• #19950 - Update NoOpResult constructors in the Engine to be public

Related Issues

• #19550 - Feature request for custom document parsing
• #19949 - Feature request for public NoOpResult constructors

Closes #1695