docs: add k-NN Model Metadata report for v2.17.0

[tkykenmt]

Summary

This PR adds documentation for the k-NN Model Metadata enhancement introduced in OpenSearch v2.17.0.

Changes

Release Report

• docs/releases/v2.17.0/features/k-nn/k-nn-model-metadata.md

Feature Report

• docs/features/k-nn/k-nn-model-metadata.md

Key Changes in v2.17.0

• Added model_version field to model metadata to track the OpenSearch version used during model training
• Changed model metadata reads to retrieve data directly from cluster metadata instead of making separate model retrieval calls
• This helps prevent null pointer exceptions by enabling version-based field presence detection
• Required for re-scoring features in disk-based vector search

Related

• Closes [enhancement] k-NN Model Metadata #409
• PR: opensearch-project/k-NN#2005