Use appendix heading for special token note

Author: sanbuphy

CLAUDE.md

@@ -54,7 +54,7 @@
 
 - 反引号（`` ` ``）只在术语第一次被定义或重点引入时使用，后续重复出现时不再加
 - 不要用 `---` 水平分割线
-- 补充性内容（如 special token）用加粗段落引入（**附录：special token**），不占用正式章节编号
+- 补充性内容（如 special token）用四级小标题引入（`#### 附录：special token`），不占用正式章节编号
 - 非概念性的过渡段落用加粗引入（**实验语料**），不作为独立 section
 
 ### 章节标题规范

notebooks-en/part1-foundation/01-tokenizer-basics.ipynb

@@ -503,7 +503,7 @@
    "id": "4ceb1a89",
    "metadata": {},
    "source": [
-    "**Appendix: Special Tokens and the Vocabulary**\n",
+    "#### Appendix: Special Tokens and the Vocabulary\n",
     "\n",
     "In practice, a Tokenizer also uses certain manually defined control symbols in addition to the tokens derived from text. These are called special tokens. The three most common ones are: `<BOS>` (Beginning of Sequence) to mark the start of a sequence, `<EOS>` (End of Sequence) to mark the end, and `<PAD>` (Padding) to pad variable-length sequences to the same length.\n",
     "\n",

notebooks/part1-foundation/01-tokenizer-basics.ipynb

@@ -575,7 +575,7 @@
    "id": "4ceb1a89",
    "metadata": {},
    "source": [
-    "**附录：special token 与词表**\n",
+    "#### 附录：special token 与词表\n",
     "\n",
     "实际的 Tokenizer 除了文本本身产生的 token 之外，还会使用一些人为约定的控制符号，称为 special token。最常见的三种：`<BOS>`（Beginning of Sequence）标记序列的开始，`<EOS>`（End of Sequence）标记序列的结束，`<PAD>`（Padding）用来补齐变长序列到相同长度。\n",
     "\n",