Adds nested knn search example

[kosabogi]

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This PR introduces a new example that demonstrates how to combine top-level and nested dense_vector fields in a single index.

Related issue: https://github.com/elastic/developer-docs-team/issues/229

[github-actions]

🔍 Preview links for changed docs:

• solutions/search/vector/knn.md

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[szabosteve]

I'm wondering if it would be better to make this guide its own subpage. This is quite complex and may make the page significantly longer – something to consider.

[leemthompo]

Good idea for a follow-up to rethink how we can break down this entire page into more manageable sub-pages —cc @kderusso

I'm pretty sure it just organically became one long append-only page because we didn't have sufficient nesting capacity in the old asciidoc system :)

[kosabogi]

Thanks for your suggestions! I’ve created an issue to address this later and improve the readability of the page.

[kderusso]

Thanks for adding this document! I've added a few suggestions.

[kderusso]

Reading the ticket closely, I wonder if we could simplify this a bit. I think the use case of "create both at once" is interesting but my take on reading this ticket is more, "we have examples of how to create regular fields but there are no good examples in the same document on how to use nested fields." It feels like only examples using nested would provide more simple examples that could be used as building blocks. Since most people who are interested in using nested fields are doing so for chunking purposes, it may make their use case a bit more clear. WDYT?

[kosabogi]

Thank you, I totally agree with your take! I updated the PR to focus only on the nested use case and removed the mixed example to keep it simpler and more relevant. What do you think about it?

[kderusso]

LGTM, thanks for iterating!

[leemthompo]

👍