[8.12](backport #3730) Add APM data stream rerouting docs

[mergify]

Closes elastic/apm-server#12375

Feel free to make big changes! Would be helpful to know if it needs better examples / more technical depth etc

---

This is an automatic backport of pull request #3730 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 1c82b46 has failed:

On branch mergify/bp/8.12/pr-3730
Your branch is up to date with 'origin/8.12'.

You are currently cherry-picking commit 1c82b46d.
  (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/observability/apm/data-streams.asciidoc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docs/en/observability/apm/ingest-pipelines.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

[carsonip]

@bmorelli25 some links are missing apm- prefix on 8.12 branch? Some PRs were missing backports?

[mergify]

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

[bmorelli25]

@bmorelli25 some links are missing apm- prefix on 8.12 branch? Some PRs were missing backports?

No, we purposely only took that PR back to 8.13 as the 8.12 docs are still in the APM Guide (not Observability Guide) and thus have apm in the URL already.