Rehome GPU accelerated vector indexing under vector search

[charlotte-hoblik]

Summary

• Adds the GPU accelerated vector indexing page under Solutions and use cases → Search → Vector search → Dense vector, matching docs-content#7796.
• Updates inbound links in the dense vector, vector search, and approximate kNN performance pages.
• Beyond the move, this PR includes light how-to polish only: an action-verb title, a Before you begin section, Next steps and Related pages, Vale/style fixes, and serverless: unavailable tagging. Configuration, Docker, monitoring, and troubleshooting are otherwise unchanged.
• This is the first PR only. The Elasticsearch mapping-reference page is still live: a follow-up PR there is needed as well.

Fixes #7796

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🔍 Preview links for changed docs

• deploy-manage/production-guidance/optimize-performance/approximate-knn-search.md
• solutions/search/vector.md
• solutions/search/vector/dense-vector.md
• solutions/search/vector/gpu-vector-indexing.md

[github-actions]

✅ Elastic Docs Style Checker (Vale)

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