docs: revise contributor onboarding (power, rewards, checklist ownership, link monitoring)

[thijsvanemmerik]

The contributor docs recommended 4 kW per DZD, never mentioned rewards management, pointed link monitoring at Grafana, and gave an account setup checklist that did not say which steps belong to the contributor and which to DZF.

• Rack and power: per-DZD figures, 2 kW across two independent feeds instead of 4 kW. The 7280CR3A-32S draws about 300 W typical and a switch-only DZD stays under 500 W with all its optics lit. Rack space is 1U in use with a second unit reserved for the edge filtering appliance.
• Rewards management: now a step in Phase 2 linking to the instructions in the contributors repository, and marked as not blocking provisioning or going live.
• Account setup order: repository access first, then keys, then rewards. The repository holds the configuration and instructions the later steps depend on.
• Checklist ownership: Phase 2 items grouped under Contributor and DZF, so the blocking handoffs are visible.
• Link monitoring: burn-in now uses the per-link page on the data portal instead of metrics.doublezero.xyz, which is Grafana. The per-link page opens on a 24 hour window and charts errors, FCS errors, discards and carrier transitions separately for each side of the link.