deploy.md: revise user story for picodata expel

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• Close #199 (closed)
• Target branch: main
• Changes should be cherry-picked to 24.2: yes
• Staging:
  • https://docs.binary.picodata.io/picodata/expel/tutorial/deploy/#expel
  • https://docs.binary.picodata.io/picodata/expel/reference/cli/#expel