chore: 更新 notebook 环境和文档

- main.ipynb: 更新 kernel 配置（Python 3.13）
- README.md: 更新项目说明
- 论文/HSTU.md: 移除（已被 PDF 版本替代）

代码/main.ipynb

@@ -365,9 +365,9 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 3",
+   "display_name": ".venv",
    "language": "python",
-   "name": "py35-paddle1.2.0"
+   "name": "python3"
   },
   "language_info": {
    "codemirror_mode": {
@@ -379,7 +379,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.10.10"
+   "version": "3.13.13"
   }
  },
  "nbformat": 4,