docs: add Explain API Support report for v3.0.0

[tkykenmt]

Summary

This PR adds documentation for the Explain API Support feature introduced in OpenSearch v3.0.0 for the k-NN plugin.

Changes

Release Report

• docs/releases/v3.0.0/features/k-nn/explain-api-support.md

Feature Report

• docs/features/k-nn/explain-api.md

Index Updates

• Updated docs/releases/v3.0.0/index.md
• Updated docs/features/index.md

Key Changes in v3.0.0

• Added Explain API support for k-NN queries using the Faiss engine
• Supports Exact, Approximate-NN (ANN), Radial, and Disk-based k-NN searches
• Provides detailed scoring explanations including raw scores and score translation formulas
• Explains why exact search was chosen over ANN in filtered scenarios

Related

• Resolves #152
• PR: opensearch-project/k-NN#2403
• Issue: opensearch-project/k-NN#875