Adds near-real-time knn parameter and vectordb_document index mode to conceptual docs

[kosabogi]

DO NOT MERGE BEFORE THE 9.5 RELEASE

Summary

This PR adds documentation for the vectordb_document index mode and near-real-time knn parameter that will be introduced in the 9.5 release.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

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🔍 Preview links for changed docs

• solutions/search/vector.md
• solutions/search/vector/knn.md

[github-actions]

Elastic Docs Style Checker (Vale)

Summary: 1 warning, 2 suggestions found

⚠️ Warnings (1): Fix when the suggestion improves clarity or correctness.

File	Line	Rule	Message
solutions/search/vector/knn.md	191	Elastic.DontUse	Don't use 'just'.

💡 Suggestions (2): Optional style improvements. Apply when helpful.

File	Line	Rule	Message
solutions/search/vector/knn.md	189	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.
solutions/search/vector/knn.md	191	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.

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