[8.16] Adds legend to LLM perf matrix (backport #6297)

[mergify]

Fixes #6285 by adding a legend to the LLM Performance Matrix that specifies which of the possible ratings are superior to which.

Preview: LLM Performance matrix

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This is an automatic backport of pull request #6297 done by Mergify.

[mergify]

Cherry-pick of a74bc31 has failed:

On branch mergify/bp/8.16/pr-6297
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit a74bc314.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   docs/AI-for-security/llm-performance-matrix.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/serverless/AI-for-security/llm-performance-matrix.asciidoc

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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