[8.13] Adds LLM performance matrix doc to ESS & serverless (backport #5286)

[mergify]

Fixes #5238 by adding a new page with a table describing how different large language models perform on various tasks within Elastic Security.

Preview: LLM performance matrix (Serverless)
LLM performance matrix (ESS)

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This is an automatic backport of pull request #5286 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of adb07fa has failed:

On branch mergify/bp/8.13/pr-5286
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Changes to be committed:
	new file:   docs/assistant/llm-performance-matrix.asciidoc
	modified:   docs/assistant/security-assistant.asciidoc
	new file:   docs/serverless/assistant/llm-performance-matrix.mdx

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/serverless/serverless-security.docnav.json

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

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