docs: Add Elasticsearch full-text search engine documentation

[claudespice]

Summary

• Document Elasticsearch as an alternative FTS engine alongside the built-in Tantivy engine
• Add dataset-level full_text_search block reference to the Spicepod datasets page
• Add new ingestion tuning parameters to the Elasticsearch vector engine page

Source PRs

• feat(elasticsearch): direct FTS engine config + index lifecycle and ingestion controls spiceai#10672 — feat(elasticsearch): direct FTS engine config + index lifecycle and ingestion controls

Changes

• website/docs/features/search/full-text.md — Added engine comparison table, Elasticsearch FTS configuration section with connection and ingestion tuning parameters, YAML anchor reuse pattern, and guidance on combining with the Elasticsearch vector engine
• website/docs/components/vectors/elasticsearch.md — Added 7 new ingestion tuning parameters to the parameters table
• website/docs/reference/spicepod/datasets.md — Added dataset-level full_text_search, full_text_search.enabled, full_text_search.engine, and full_text_search.params reference sections

Test plan

• Verified parameter names and defaults match source code ParameterSpec definitions
• Links are valid
• Version references are correct (vNext only — new feature)

[github-actions]

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[github-actions]

🚀 deployed to https://61218576.spiceai-org-website.pages.dev