KIP-848 migration guide

[Emanuele Sabellico (emasab)]

Markdown document is in md-docs as docs is already used for automatic reference documentation generation.

[Copilot]

Pull Request Overview

This PR adds a comprehensive migration guide for KIP-848, the next generation consumer group rebalance protocol in confluent-kafka-javascript 1.6.0. The guide provides detailed information about migrating from the classic protocol to the new consumer protocol.

• Introduces a detailed migration guide document explaining KIP-848 features and contract changes
• Updates the changelog to announce KIP-848 general availability with reference to the migration guide

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
md-docs/kip-848-migration-guide.md	New comprehensive migration guide covering KIP-848 protocol changes, configuration updates, and migration steps
CHANGELOG.md	Adds KIP-848 GA announcement with link to migration guide

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