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/document-processing/README.md

@@ -0,0 +1,11 @@
+# 文档处理
+
+文档处理负责聊天附件和知识库导入中的文本提取、格式转换与本地 OCR。
+
+## 文档导航
+
+| 文档 | 权威职责 |
+| --- | --- |
+| [文档解析与本地 OCR PRD](./prd.md) | 产品范围、解析策略、OCR 模型管理和验收 |
+
+当前尚无独立 User Stories、功能逻辑、UI、技术设计和进度文档。