Enable markdownlint no-duplicate-header (#27500) (#27506)

Backport #27500 by @silverwind

Duplicate headers in a single Markdown document are problemlematic
because the auto-generated links won't be stable. Enable this rule with
no exceptions which is also the default of `markdownlint`. For example:

```md
# A
## Example
# B
## Example
```
Docasaurus will generated `example` and `example-1` links for this. If
the first heading is altered, the link `example` will unexpectedly move
to the second example heading.

Ref: https://github.com/go-gitea/gitea/pull/27461#discussion_r1347987659

Co-authored-by: silverwind <me@silverwind.io>

docs/content/administration/cmd-embedded.en-us.md

@@ -50,7 +50,7 @@ a special meaning for your command shell.
 
 If no pattern is provided, all files are listed.
 
-### Example
+### Example: Listing all embedded files
 
 Listing all embedded files with `openid` in their path:
 
@@ -101,7 +101,7 @@ When Gitea is upgraded to a new version (by replacing the executable), many of t
 embedded files will suffer changes. Gitea will honor and use any files found
 in the `custom` directory, even if they are old and incompatible.
 
-### Example
+### Example: Extracting mail templates
 
 Extracting mail templates to a temporary directory:
 

docs/content/administration/cmd-embedded.zh-cn.md

@@ -43,7 +43,7 @@ gitea embedded list [--include-vendored] [patterns...]
 
 如果未提供模式，则列出所有文件。
 
-### 示例
+### 示例：列出所有嵌入文件
 
 列出所有路径中包含 `openid` 的嵌入文件：
 
@@ -83,7 +83,7 @@ gitea [--config {file}] embedded extract [--destination {dir}|--custom] [--overw
 
 请确保**只提取需要自定义的文件**。位于 `custom` 目录中的文件不会受到 Gitea 的升级过程的影响。当 Gitea 升级到新版本（通过替换可执行文件）时，许多嵌入文件将发生变化。Gitea 将尊重并使用在 `custom` 目录中找到的任何文件，即使这些文件是旧的和不兼容的。
 
-### 示例
+### 示例：提取邮件模板
 
 将邮件模板提取到临时目录：