docs: organize product design by feature

Define repository and feature-scoped documentation rules for PRDs, User Stories, logic, UI, technical design, and progress evidence.

Move feature-owned documents under docs/features, add one entry point per feature, remove the redundant docs index, and update all affected cross-references. Expand the local tool environment design with explicit product, story, logic, UI, technical, and progress documents.

docs/features/parallel-experiments/README.md

@@ -0,0 +1,11 @@
+# 并行实验
+
+并行实验定义多个候选方案、模型或执行分支的受控比较、评估与结果选择。
+
+## 文档导航
+
+| 文档 | 权威职责 |
+| --- | --- |
+| [并行实验工作台 PRD](./prd.md) | 实验对象、工作台流程、评估和验收 |
+
+当前尚无独立 User Stories、功能逻辑、UI、技术设计和进度文档。