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

14_etcd/cluster.md

@@ -1,4 +1,4 @@
-## etcd 集群
+## 14.3 etcd 集群
 
 下面我们使用 [Docker Compose](../../10_compose/README.md) 模拟启动一个 3 节点的 `etcd` 集群。