docs: add filename usage for code blocks

fix #43

content/1.guide/3.writing.md

@@ -130,6 +130,18 @@ And you can use inline components to props to `span`or to use an icon.
 More information about the MDC syntax can be found in the [MDC documentation](https://content.nuxtjs.org/guide/writing/mdc#mdc-syntax).
 ::
 
+::alert{type="info"}
+For code block, we recommend you to add a filename or the terminal name to give a better understanding to the reader.
+
+```
+\```md [index.md]
+<!-- ... -->
+\```
+```
+
+You can see a text between `[` and `]` in the code block. This is the filename.
+::
+
 #### Mermaid
 
 You can use [Mermaid](https://mermaid-js.github.io/mermaid/#/) to create diagrams.