[8.7] [DOCS] Add frontmatter for migration (backport #2402)

[mergify]

This is an automatic backport of pull request #2402 done by Mergify.
Cherry-pick of 7b94f76 has failed:

On branch mergify/bp/8.7/pr-2402
Your branch is up to date with 'origin/8.7'.

You are currently cherry-picking commit 7b94f76e.
  (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/stack/ml/anomaly-detection/ml-ad-explain.asciidoc
	modified:   docs/en/stack/ml/anomaly-detection/ml-ad-finding-anomalies.asciidoc
	modified:   docs/en/stack/ml/anomaly-detection/ml-ad-job-types.asciidoc
	modified:   docs/en/stack/ml/df-analytics/ml-dfa-classification.asciidoc
	modified:   docs/en/stack/ml/df-analytics/ml-dfa-outlier-detection.asciidoc
	modified:   docs/en/stack/ml/df-analytics/ml-dfa-overview.asciidoc
	modified:   docs/en/stack/ml/df-analytics/ml-dfa-regression.asciidoc
	modified:   docs/en/stack/ml/df-analytics/ml-how-dfa-works.asciidoc
	modified:   docs/en/stack/ml/get-started/ml-getting-started.asciidoc
	modified:   docs/en/stack/ml/machine-learning-intro.asciidoc
	modified:   docs/en/stack/ml/nlp/ml-nlp-classify-text.asciidoc
	modified:   docs/en/stack/ml/nlp/ml-nlp-extract-info.asciidoc
	modified:   docs/en/stack/ml/nlp/ml-nlp-model-ref.asciidoc
	modified:   docs/en/stack/ml/nlp/ml-nlp.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   docs/en/stack/ml/nlp/ml-nlp-deploy-models.asciidoc
	deleted by us:   docs/en/stack/ml/nlp/ml-nlp-elser.asciidoc
	deleted by us:   docs/en/stack/ml/nlp/ml-nlp-limitations.asciidoc

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

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