添加docfx支持，以便于将文档生成为带目录的PDF文档

add docfx support, make PDF generate with Table of Content

Author: jiankian

.gitignore

@@ -1,2 +1,3 @@
 book
 .idea
+_site

docfx.json

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+{
+  "$schema": "https://raw.githubusercontent.com/dotnet/docfx/main/schemas/docfx.schema.json",
+  "build": {
+    "content": [
+      {
+        "files": [
+          "**/*.{md,yml}"
+        ],
+        "exclude": [
+          "_site/**"
+        ]
+      }
+    ],
+    "resource": [
+      {
+        "files": [
+          "images/**"
+        ]
+      }
+    ],
+    "output": "_site",
+    "template": [
+      "default",
+      "modern"
+    ],
+    "globalMetadata": {
+      "_appName": "\u901A\u8FC7\u4F8B\u5B50\u5B66\u4E60Rust - by docfx",
+      "_appTitle": "\u901A\u8FC7\u4F8B\u5B50\u5B66\u4E60Rust - by docfx",
+      "_enableSearch": true,
+      "pdf": true
+    }
+  }
+}

index.md

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+---
+_layout: landing
+---
+
+# This is the **HOMEPAGE**.
+
+Refer to [Markdown](http://daringfireball.net/projects/markdown/) for how to write markdown files.
+
+## Quick Start Notes:
+
+1. Add images to the *images* folder if the file is referencing an image.