[Docs] ESQL - Add _source details in the search tutorial

[carlosdelest]

This PR adds a section to the ES|QL search tutorial that includes when it would be useful to use _source instead of selecting individual fields.

@jimczi how does this look like from the technical PoV?

@leemthompo I'm thinking to add some / all of this to the metadata fields section. WDYT? Should we add all details there and put a link to that on the search tutorial?

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[github-actions]

🔍 Preview links for changed docs

• docs/reference/query-languages/esql/esql-metadata-fields.md
• docs/reference/query-languages/esql/esql-search-tutorial.md

[github-actions]

ℹ️ Important: Docs version tagging

👋 Thanks for updating the docs! Just a friendly reminder that our docs are now cumulative. This means all 9.x versions are documented on the same page and published off of the main branch, instead of creating separate pages for each minor version.

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Expand for a quick overview

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✅ At the page level to indicate which products/deployments the content applies to (mandatory)
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[leemthompo]

As you suggested Carlos, I'd lean towards putting the "when to use / when not to use" lists in the metadata fields page and linking there from the final sentence in the tutorial:

"Using _source or selecting fields are both valid options, but can have performance and data format implications that you should consider based on your use case. To learn more, refer to Sort results by search score.

You could keep a brief line in the tutorial about "You should consider using _source when foo bar baz"

[carlosdelest]

I added the original criteria in the metadata page, and summarized in the search tutorial. LMKWYT @leemthompo @jimczi !

[leemthompo]

LGTM

[iverase]

geo fields suffer the same, maybe would be worthy to add them here, in particular big geo_shape fields.