Fix documentation build problems because of MDX syntax conflicts (#30744)

Documentation building has encountered a problem like below. This is
because MDX syntax doesn't allow `{customPath}`, we have to use
\`{customPath}\`

```
Error: Can't render static file for pathname "/next/administration/config-cheat-sheet"
            at generateStaticFile (/workspace/gitea/gitea-docusaurus/node_modules/@docusaurus/core/lib/ssg.js:119:15)
            at runNextTicks (node:internal/process/task_queues:60:5)
            at process.processImmediate (node:internal/timers:449:9)
            at async /workspace/gitea/gitea-docusaurus/node_modules/p-map/index.js:57:22 {
          [cause]: ReferenceError: CustomPath is not defined
              at _createMdxContent (server.bundle.js:4406:106)
              at MDXContent (server.bundle.js:10745:8)
              at Uc (server.bundle.js:264171:44)
              at Xc (server.bundle.js:264173:253)
              at Z (server.bundle.js:264179:89)
              at Yc (server.bundle.js:264182:98)
              at $c (server.bundle.js:264181:140)
              at Z (server.bundle.js:264179:345)
              at Xc (server.bundle.js:264177:231)
              at Z (server.bundle.js:264179:89)
```

Author: lunny

docs/content/administration/config-cheat-sheet.en-us.md

@@ -214,9 +214,9 @@ The following configuration set `Content-Type: application/vnd.android.package-a
 - `SITEMAP_PAGING_NUM`: **20**: Number of items that are displayed in a single subsitemap.
 - `GRAPH_MAX_COMMIT_NUM`: **100**: Number of maximum commits shown in the commit graph.
 - `CODE_COMMENT_LINES`: **4**: Number of line of codes shown for a code comment.
-- `DEFAULT_THEME`: **gitea-auto**: Set the default theme for the Gitea installation, custom themes could be provided by "{CustomPath}/public/assets/css/theme-*.css".
+- `DEFAULT_THEME`: **gitea-auto**: Set the default theme for the Gitea installation, custom themes could be provided by `{CustomPath}/public/assets/css/theme-*.css`.
 - `SHOW_USER_EMAIL`: **true**: Whether the email of the user should be shown in the Explore Users page.
-- `THEMES`: **_empty_**: All available themes by "{CustomPath}/public/assets/css/theme-*.css". Allow users select personalized themes.
+- `THEMES`: **_empty_**: All available themes by `{CustomPath}/public/assets/css/theme-*.css`. Allow users select personalized themes.
 - `MAX_DISPLAY_FILE_SIZE`: **8388608**: Max size of files to be displayed (default is 8MiB)
 - `AMBIGUOUS_UNICODE_DETECTION`: **true**: Detect ambiguous unicode characters in file contents and show warnings on the UI
 - `REACTIONS`: All available reactions users can choose on issues/prs and comments

docs/content/administration/config-cheat-sheet.zh-cn.md

@@ -212,9 +212,9 @@ menu:
 - `SITEMAP_PAGING_NUM`: **20**: 在单个子SiteMap中显示的项数。
 - `GRAPH_MAX_COMMIT_NUM`: **100**: 提交图中显示的最大commit数量。
 - `CODE_COMMENT_LINES`: **4**: 在代码评论中能够显示的最大代码行数。
-- `DEFAULT_THEME`: **gitea-auto**: 在Gitea安装时候设置的默认主题，自定义的主题可以通过 "{CustomPath}/public/assets/css/theme-*.css" 提供。
+- `DEFAULT_THEME`: **gitea-auto**: 在Gitea安装时候设置的默认主题，自定义的主题可以通过 `{CustomPath}/public/assets/css/theme-*.css` 提供。
 - `SHOW_USER_EMAIL`: **true**: 用户的电子邮件是否应该显示在`Explore Users`页面中。
-- `THEMES`:  **_empty_**: 所有可用的主题（由 "{CustomPath}/public/assets/css/theme-*.css" 提供）。允许用户选择个性化的主题，
+- `THEMES`:  **_empty_**: 所有可用的主题（由 `{CustomPath}/public/assets/css/theme-*.css` 提供）。允许用户选择个性化的主题，
 - `MAX_DISPLAY_FILE_SIZE`: **8388608**: 能够显示文件的最大大小（默认为8MiB）。
 - `REACTIONS`: 用户可以在问题（Issue）、Pull Request（PR）以及评论中选择的所有可选的反应。
     这些值可以是表情符号别名（例如：:smile:）或Unicode表情符号。