Add Vector Database project type links to vector-related search pages

[kosabogi]

This PR adds Vector Database project type guidance to the vector, semantic, hybrid, RAG, and multimodal search pages.

Fixes #8325

Page	What changed
Search use case	Introduces the two Serverless search project types. Links vector terms to the Vector search glossary. Recommends Vector Database when the workload is mainly vector-based.
Semantic search for text	Adds a Serverless project-type recommendation in the intro.
Semantic search with semantic_text	Recommends Vector Database when embeddings are primary; Elasticsearch project for ML-node or mainly lexical search.
Semantic search quickstart	Extends prerequisites with the same Serverless project-type guidance.
Hybrid search	Adds project-type guidance in the intro.
Hybrid search with semantic_text	Adds project-type guidance before requirements.
RAG	Recommends Vector Database for embedding-driven retrieval.
Semantic search with ELSER	Notes that this tutorial needs ML nodes (Elasticsearch project), and points to semantic_text otherwise.
Multimodal search	Adds project-type guidance in the intro.
Tutorial: Build multimodal search	Specifies Vector Database in Before you begin, plus a trial signup link.

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🔍 Preview links for changed docs

• solutions/search.md
• solutions/search/get-started/semantic-search.md
• solutions/search/hybrid-search.md
• solutions/search/hybrid-semantic-text.md
• solutions/search/multimodal-search.md
• solutions/search/multimodal-search/multimodal-search-tutorial.md
• solutions/search/rag.md
• solutions/search/semantic-search.md
• solutions/search/semantic-search/semantic-search-elser-ingest-pipelines.md
• solutions/search/semantic-search/semantic-search-semantic-text.md

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