[8.11](backport #3997) fix: replace ndjson.org with ndjson spec

[mergify]

the original website expired and it's currently serving malicious content.

Closes #3981

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This is an automatic backport of pull request #3997 done by Mergify.

[mergify]

Cherry-pick of 698d4e0 has failed:

On branch mergify/bp/8.11/pr-3997
Your branch is up to date with 'origin/8.11'.

You are currently cherry-picking commit 698d4e00.
  (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/en/apm-server/api-events.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/en/serverless/apm/apm-server-api/api-events.mdx

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

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @kruskall? 🙏