docs: add SSO troubleshooting hub

[saif-at-scalekit]

Customer signal

• Support triage 2026-06 (Pylon + Slack harvest)
• Recurring SSO IdP misconfiguration questions

Symptom

Developers find error codes in sso/reference/sso-integration-errors.mdx but the page is noindex and not linked from a journey troubleshooting entry.

Proposed docs placement

Content type: How-to hub
File path: src/content/docs/sso/guides/troubleshooting.mdx
Journey phase: Modular SSO → Guides (before Go live)
Escalation: medium (new page + sidebar)

Acceptance criteria

• Symptom → fix sections for common IdP errors
• Link to sso-integration-errors reference
• Cross-link from sso/guides/test-sso.mdx and launch checklist
• Remove noindex from error reference or link prominently from hub
• Build + format pass