Add Synthetic _source callout to APM + UP docs

[bmorelli25]

Preview:

• https://observability-docs_bk_4657.docs-preview.app.elstc.co/guide/en/observability/master/apm-data-streams.html
• https://observability-docs_bk_4657.docs-preview.app.elstc.co/guide/en/observability/master/profiling-manage-storage.html

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @bmorelli25? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.