docs: refactor document folder structure.

docs/documents/articles/ecs/toc.yml

@@ -0,0 +1,14 @@
+- name: Entity Component System
+  items:
+  - name: Getting Started
+    href: getting-started.md
+  - name: ECS Concepts
+    href: ecs-concepts.md
+  - name: Components
+    href: components.md
+  - name: Systems
+    href: systems.md
+  - name: ECS Workflows
+    href: ecs-workflows.md
+  - name: Performance
+    href: performance.md