docs(docs): add strategy, coding, and system design documentation hierarchy

docs/content/02-guides/02-system-design/stream-processing.mdx

@@ -0,0 +1,12 @@
+---
+title: 'Stream Processing'
+description: 'Sliding window as a bounded stream with an eviction policy.'
+sidebar:
+  order: 3
+---
+
+:::warning[Under construction]
+This page is a scaffold. Notes, canonical problems, and template code are not written yet.
+:::
+
+**Coding family:** Contiguous Linear.