Dedupe and clarify "Search your data" (#213)

* Dedupe and clarify "Search your data"

> Important: These docs now avoid duplicating content from core ES documentation to reduce maintenance overhead and prevent documentation drift. Instead, they focus on serverless-specific content and refer to core docs for implementation details.

These docs were manually duplicated a looong time ago and are now incomplete and out of date.

- Removed duplicated implementation guides
- References core ES docs
- Added tip for "bring your own vectors" tutorial

- Removed duplicated implementation details
- Points to core ES docs
- Marked as duplicate/removed from navigation

- Added Retrievers section
- Simplified Query DSL explanation
- References core ES docs

- Rewritten for serverless context
- Links to quickstart tutorials and notebooks
- Points to core docs for concepts

* Delete synonyms page (redirect is already in place)

* fixes per review

* Apply suggestions from code review

Co-authored-by: shainaraskas <[email protected]>

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Co-authored-by: shainaraskas <[email protected]>

serverless/index-serverless-elasticsearch.asciidoc

@@ -37,7 +37,6 @@ include::./pages/ingest-your-data-ingest-data-through-integrations-beats.asciido
 
 include::./pages/search-your-data.asciidoc[leveloffset=+2]
 include::./pages/search-your-data-the-search-api.asciidoc[leveloffset=+3]
-include::./pages/search-with-synonyms.asciidoc[leveloffset=+3]
 include::./pages/knn-search.asciidoc[leveloffset=+3]
 include::./pages/search-your-data-semantic-search.asciidoc[leveloffset=+3]