docs: add docs for file imports

docs/tutorialkit.dev/src/content/docs/guides/creating-content.mdx

@@ -144,3 +144,25 @@ src/templates
     │   # Overrides "index.js" from "shared-template"
     └── index.js
 ```
+
+## Importing files
+
+You can import files from your code template, `_files` and `_solution` folder using the `file` or `solution` shortcode. This shortcode inserts the content of the specified file directly into your lesson content.
+
+- `file` shortcode is used to reference files from the lesson `_files` or code template folder.
+- `solution` shortcode is used to reference files from the lesson `_solution` folder.
+
+For example, the following code will insert the content of the `box.css` file from the `_files` folder:
+
+````md "file"
+```file:/box.css
+```
+````
+
+```css
+.box {
+  width: 100px;
+  height: 100px;
+  background-color: red;
+}
+```