[main] (forwardport #18540) Doc: Fix links in Logstash-to-serverless 8.18

[mergify]

Fix asciidoc cross-doc links

PREVIEW: https://logstash_bk_18540.docs-preview.app.elstc.co/guide/en/logstash/8.18/logstash-to-elasticsearch-serverless.html

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This is an automatic backport of pull request #18540 done by Mergify.

[mergify]

Cherry-pick of a0cef97 has failed:

On branch mergify/bp/main/pr-18540
Your branch is up to date with 'origin/main'.

You are currently cherry-picking commit a0cef977.
  (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)

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

no changes added to commit (use "git add" and/or "git commit -a")

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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