Add npm command for legacy Node.

[xavierlepretre]

This PR makes more explicit how to use the legacy command line to run serve.

R1: Internal review (B9lab)

• Technical review
  • [ ] requested
  • [ ] completed
• Language review
  • [ ] requested
  • [ ] completed
• Technical platform review (remove if not required)
  • [x] requested
  • [ ] completed

R2: External review

• Technical review
  • [ ] requested
  • [ ] completed
• Language review
  • [ ] requested
  • [ ] completed

R3: Internal QA review (B9lab)

• Technical review
  • [ ] requested
  • [ ] completed
• Language review
  • [ ] requested
  • [ ] completed

R4: Final external QA review

• Technical review
  • [ ] requested
  • [ ] completed
• Language review
  • [ ] requested
  • [ ] completed

RC: Release Candidate

• [ ] Ready to be merged