docs: guide for adding custom grammars for syntax highlighting

Author: ashour

docs/content/4.api/3.configuration.md

@@ -317,6 +317,36 @@ export default defineNuxtConfig({
 })
 ```
 
+If you wish to add highlighting for an unsupported language, you can do so by loading the grammar file for the language.
+
+```ts
+import { readFileSync } from 'fs'
+
+export default defineNuxtConfig({
+  content: {
+    highlight: {
+      preload: [
+        {
+          id: 'gdscript',
+          scopeName: 'source.gdscript',
+          aliases: ['gdscript', 'gd'], // Use to mark code blocks in Markdown
+          grammar: JSON.parse(
+            readFileSync(
+              // Place the language grammar file somewhere in your project
+              './shiki/languages/gdscript.tmLanguage.json'
+            ).toString()
+          ),
+        },
+      ],
+    },
+  },
+})
+
+```
+
+Read more about adding languages in the [Shiki documentation](https://github.com/shikijs/shiki/blob/main/docs/languages.md#adding-grammar).
+
+
 ## `yaml`
 
 - Type: `false | Object`{lang=ts}