style(docs): standardize heading formats and numbering

- Enforce Level 1-3 structural numbering based on SUMMARY.md hierarchy
- Remove structural numbering from Level 4+ headings
- Eliminate single child headings by converting to bold text
- Auto-inject summary text for headings with multiple children missing intro text
- Exclude Appendix chapters from structural numbering
- Avoid modifying code block contents
- Add script to detect non-standard English usage in headers

17_security/kernel_ns.md

@@ -1,4 +1,4 @@
-## 内核命名空间
+## 17.1 内核命名空间
 
 命名空间 (Namespace) 是 Linux 容器隔离的基础，它确保了容器内的进程无法干扰主机或其他容器。