[8.10] fix: remove ndjson.org (backport #5376)

[mergify]

Fixes https://github.com/elastic/security-docs-internal/issues/20. The original website expired and it's currently serving malicious content.

Previews

ESS:

• Investigate events in Timeline | Export and import Timelines
• About Timeline templates | Export and import Timeline templates

Serverless:

• Investigate events in Timeline | Export and import Timelines
• About Timeline templates | Export and import Timeline templates

Backporting notes

• Mergify should be able to create all 8.x backports, and the 7.14-7.17 backports. To create backport PRs for 7.9-7.13, we'll need to use the backport tool.
• For backports earlier than 7.17, we’ll need to manually remove the serverless folder & files from the backport PRs. Our GitHub action will do this automatically for backports to 7.17 and later.

  ---

  This is an automatic backport of pull request fix: remove ndjson.org #5376 done by Mergify.

[mergify]

Cherry-pick of 5beed29 has failed:

On branch mergify/bp/8.10/pr-5376
Your branch is up to date with 'origin/8.10'.

You are currently cherry-picking commit 5beed294.
  (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/events/timeline-templates.asciidoc
	modified:   docs/events/timeline-ui-overview.asciidoc
	modified:   docs/getting-started/siem-ui.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/serverless/investigate/timeline-templates-ui.mdx
	deleted by us:   docs/serverless/investigate/timelines-ui.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

[github-actions]

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