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

docs/content/02-guides/01-coding/08-relational/index.mdx

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+---
+title: 'Relational'
+description: 'Arbitrary edges; you must track visited. Trees are acyclic connected graphs with less bookkeeping.'
+sidebar:
+  order: 1
+  label: Introduction
+---
+
+:::warning[Under construction]
+This page is a scaffold. Notes, canonical problems, and template code are not written yet.
+:::
+
+## Patterns
+
+- [DFS on Graphs](/guides/coding/relational/graph-dfs) — Exhaustive reachability: recursion plus a visited set.
+- [BFS on Graphs](/guides/coding/relational/graph-bfs) — Frontier expansion: shortest path in unweighted graphs, always.
+- [Topological Sort](/guides/coding/relational/topological-sort) — Reversed DFS post-order, or Kahn's in-degree queue over a DAG.
+- [Union-Find](/guides/coding/relational/union-find) — Incremental connectivity without traversal.
+
+**Systems anchor:** Topological sort is dependency resolution; union-find is partition detection; BFS is gossip propagation.