feat: 初始化手写数字识别项目

包含两层神经网络的完整实现（纯 numpy）：
- 核心模块：dataset / model / optimizer / trainer
- 14章深度学习入门小册子（book/）
- VitePress 网站配置（支持 GitHub Pages & Vercel 部署）
- GitHub Actions 自动部署工作流

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: leonyangdev

.github/workflows/deploy-book.yml

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+name: Deploy Book to GitHub Pages
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'book/**'
+      - '.github/workflows/deploy-book.yml'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: pages
+  cancel-in-progress: false
+
+jobs:
+  build:
+    name: Build VitePress
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+          cache-dependency-path: book/package.json
+
+      - name: Install dependencies
+        working-directory: book
+        run: npm install
+
+      - name: Build VitePress
+        working-directory: book
+        env:
+          # 仓库名改成你在 GitHub 上起的名字，例如 digit-recognizer
+          VITEPRESS_BASE: /digit-recognizer/
+        run: npm run build
+
+      - name: Upload Pages artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          path: book/.vitepress/dist
+
+  deploy:
+    name: Deploy to GitHub Pages
+    needs: build
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@v4

.gitignore

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+# Python
+__pycache__/
+*.py[cod]
+*.egg-info/
+.venv/
+venv/
+dist/
+build/
+
+# 数据文件（太大，不入库）
+data/
+outputs/
+
+# Jupyter
+.ipynb_checkpoints/
+
+# macOS
+.DS_Store
+
+# VitePress 构建产物
+book/node_modules/
+book/.vitepress/cache/
+book/.vitepress/dist/