add content to the custom extension page

docs/src/docPages/guide/custom-extensions.md

@@ -1,7 +1,31 @@
 # Custom Extensions
 
+Let’s extend tiptap with a custom extension!
+
 ## Option 1: Change defaults
 
+Let’s say you want to change the keyboard shortcuts for the bullet list. You should start by looking at [the source code of the `Bold` extension](https://github.com/ueberdosis/tiptap-next/blob/main/packages/extension-bold/index.ts) and find the default you’d like to change. In that case, the keyboard shortcut.
+
+```js
+// 1. Import the extension
+import BulletList from '@tiptap/extension-bullet-list'
+
+// 2. Overwrite the keyboard shortcuts
+const CustomBulletList = new Node()
+  .keys(({ editor }) => ({
+    'Mod-l': () => editor.bulletList(),
+  }))
+  .create() // Don’t forget that!
+
+// 3. Add the custom extension to your editor
+new Editor({
+  extensions: [
+      CustomBulletList(),
+      // …
+  ]
+})
+```
+
 ## Option 2: Extend existing extensions
 
 ## Option 3: Start from scratch