[ML] Updates Semantic search section

[kosabogi]

Overview

This PR restructures the Semantic search section of the serverless documentation.
To avoid duplication, we removed redundant content from the serverless documentation and added links to existing tutorials in the Elasticsearch documentation wherever possible.

The Semantic search page now contains short descriptions, introductions and links to the following guides:

• Semantic search
• Semantic search with the inference API
• Semantic search with semantic_text
• Semantic search with deployed model

Related issues

#61
#156

Preview

Semantic search

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[leemthompo]

Thanks! Few adjustments needed but we should be able to get this merged today :)

[leemthompo]

👌 🚢 🙇