Include Pinned Retriever in 8.19 Documentation

[mridula-s109]

Summary of changes

• Added details about the Pinned Retriever to retrievers-overview .
• Ensured that only the intended documentation file was modified, with no unrelated changes.

Test output

[github-actions]

Documentation preview:

• ✨ Changed pages

[elasticsearchmachine]

Pinging @elastic/search-eng (Team:SearchOrg)

[elasticsearchmachine]

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

[Copilot]

Pull Request Overview

This PR updates the search documentation to include details for the new Pinned Retriever in version 8.19.

• Added an entry for Pinned Retriever in the retrievers overview file.
• Included a new example section with parameter details and usage examples for the pinned retriever in the examples file.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docs/reference/search/search-your-data/retrievers-overview.asciidoc	Added a new bullet referencing the Pinned Retriever with a brief explanation.
docs/reference/search/search-your-data/retrievers-examples.asciidoc	Introduced a new example section for the Pinned Retriever with detailed parameter and usage instructions.

[mridula-s109]

@kderusso Have made changes related to the index usage as discussed in the call yesterday.

[kderusso]

Let's please follow the standard that we use for other docs. Please reach out to @leemthompo if you have questions on where you should put stuff as well, thanks.

[mridula-s109]

Let's please follow the standard that we use for other docs. Please reach out to @leemthompo if you have questions on where you should put stuff as well, thanks.

Thanks for your comments, Have done the same!

[kderusso]

LGTM, thanks!