docs: add SRE troubleshooting playbooks

[martinraumann]

Summary

• Adds SRE-oriented troubleshooting playbooks for NICo stuck objects and production operations.
• Splits the SRE troubleshooting guide into focused pages for state machine debugging, diagnostic tools, DPU provisioning, host ingestion, health alerts, connectivity, instance/fabric issues, and site-controller health.
• Wires the new pages into the existing Operations (Day 2) -> Playbooks -> Stuck Objects docs navigation.

Jira

• FORGE-8239

Notes

• Based on David Mateer's SRE troubleshooting material.
• Command examples were scrubbed against the current NICo CLI where possible.
• Historical carbide_* names are kept only where they are current metric names.

Validation

• npx fern-api@5.40.1 check passed with 0 errors.
• npx fern-api@5.40.1 check --warnings shows 2 existing repo/config warnings: unauthenticated redirects check skipped, and accent color contrast ratio.
• git diff --check passed.

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