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

docs/content/02-guides/01-coding/05-nodal-linear/index.mdx

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+---
+title: 'Nodal Linear'
+description: 'O(1) splice, no random access. Everything is pointer surgery.'
+sidebar:
+  order: 1
+  label: Introduction
+---
+
+:::warning[Under construction]
+This page is a scaffold. Notes, canonical problems, and template code are not written yet.
+:::
+
+## Patterns
+
+- [Linked List](/guides/coding/nodal-linear/linked-list) — Fast/slow runners, reversal, and dummy-head splicing cover it all.
+
+**Systems anchor:** LRU cache = hashmap + doubly-linked list, the one hybrid that appears constantly.