feat(new): Add new Qwen3 and Mixbread rerankers

[pwoznic]

Adds content for the new Qwen3 and Mixbread rerankers.

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[Copilot]

Pull Request Overview

This PR adds support for two new neural reranking models (Qwen3 and Mixbread) to the Vectara platform, with Qwen3 becoming the new default reranker for SaaS deployments.

• Introduces Qwen3 and Mixbread rerankers as new options for improving search result relevance
• Updates documentation to reflect the new default reranker and provide guidance on model selection
• Adds comprehensive examples and best practices for using the new reranking models

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
www/docs/release-notes.mdx	Adds release notes announcing the new Qwen3 and Mixbread rerankers
www/docs/learn/vectara-multi-lingual-reranker.md	Updates content to reflect new reranker options and removes outdated superlatives
www/docs/learn/knee-reranking.md	Adds example showing knee reranking compatibility with new Qwen3 reranker
www/docs/api-reference/search-apis/reranking.md	Restructures reranker documentation with comprehensive table and adds usage examples for new models

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