docs: document escalation contacts and user management

[thijsvanemmerik]

Updates the OPS Management contributor docs to cover features and behavior that were missing or wrong:

• Escalation Contacts: on-call contacts (a person or a team such as a NOC), availability (24/7 or weekly slots), contact methods, coverage gaps, rotation windows, and visibility (DoubleZero only / Everybody / Some contributors).
• User Management: adding team members with read or read-write access.
• Maintenance severity: documents that severity is required for maintenance tickets, not just incidents, correcting the previous page which omitted it.

Also updates Onboarding to point at the new Settings (gear) menu, which now holds API Key Management, User Management, and Escalation Contacts.