All you need to do is switch to HTML mode, then add a <details> tag with another <summary> tag to say what it's about and then any other tags inside to hide.
Example:
<details>
<summary>Game Walkthrough</summary>
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.
</details>