docs: add canonical IA landing + first-pass core pages

docs/10-troubleshooting.md

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+# Troubleshooting
+
+This is the high-level troubleshooting entry point (minimal churn).
+
+- Deep-dive troubleshooting: [docs/troubleshooting/README.md](troubleshooting/README.md)
+
+## Quick triage
+
+### Symptom: frontend loads but shows API errors
+- Confirm `NEXT_PUBLIC_API_URL` points to a reachable backend.
+- Check backend `/healthz`.
+
+### Symptom: frontend keeps redirecting / Clerk errors
+- If you are running locally without Clerk, ensure `NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY` is **unset/blank**.
+- See: [repo README Clerk note](../README.md#note-on-auth-clerk).
+
+### Symptom: backend 5xx
+- Check DB connectivity (`DATABASE_URL`) and migrations.
+- Check backend logs.
+
+## Next
+- Promote the most common issues from `docs/troubleshooting/README.md` into this page once we see repeated incidents.