docs(doc-entries): Show examples as code blocks

Directive examples sat bare in the doc bodies, so rendering them as
markdown executed the directives: live components, missing-file
fetches, and style side effects on the dialog. Wrap each example in
a markdown fence, matching the convention already used by md-table.

src/doc-entries/md-font.md

@@ -9,18 +9,25 @@ title: 字体组件
 **语法：** `:md-font[字体名]{选项}`
 
 **Google 字体：**
+```markdown
 :md-font[Noto Sans SC]{source="google"}
+```
 
 **Emfont 字体：**
+```markdown
 :md-font[Source Han Serif]{source="emfont" weight="700"}
+```
 
 **本地字体（默认）：**
+```markdown
 :md-font[STSong]
+```
 
 **设置文字颜色：**
+```markdown
 :md-font[Noto Sans SC]{source="google" color="#1a1a2e"}
-
 :md-font[Noto Sans SC]{source="google" weight="700" color="crimson"}
+```
 
 字体和颜色将应用到整个文档卡片。