Add privacy-safe demand forecasting case study

Synthetic monthly SKU/region demand notebook comparing a trimmed-mean
baseline against gradient boosting, with a gated TimesFM build-vs-buy
comparison. Includes rolling-origin validation, executive narrative,
architecture diagram, one-page PDF, and LinkedIn draft.

No company data, credentials, or private implementation details.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/architecture.mmd

@@ -0,0 +1,16 @@
+flowchart LR
+    A[Synthetic monthly demand] --> B[Prepare dates and features]
+    B --> C[Trimmed-mean baseline]
+    B --> D[Gradient-boosting model]
+    C --> E[Rolling validation]
+    D --> E
+    E --> F[Select by sMAPE]
+    F --> G[12-month forecast]
+
+    H[Manual trigger today] -.-> B
+    I[Future Airflow boundary] -.-> B
+
+    subgraph Delivery["Delivery ownership"]
+      J[Business question] --> K[Reproducible evidence]
+      K --> L[Review and adoption]
+    end