docs: add Modular SCIM troubleshooting guide

[saif-at-scalekit]

Customer signal

• Support triage 2026-06
• SCIM provisioning failures lack a consolidated troubleshooting entry (unlike MCP Auth)

Proposed docs placement

Content type: How-to / troubleshooting
File path: src/content/docs/directory/guides/troubleshooting.mdx
Journey phase: Modular SCIM → Go live
Escalation: medium

Link existing cookbook: cookbooks/scim-seat-limit-enforcement.mdx

Acceptance criteria

• Common sync/provisioning failure symptoms
• Sidebar entry under Go live
• Cross-link from launch checklist and webhooks reference