Social SDK: Updated Unity Install Instructions (discord#7544)

Unity SDK download is now a zip file, so the installation instructions
have now changed.

Author: markmandel

docs/discord-social-sdk/getting-started/using-unity.mdx

@@ -50,10 +50,11 @@ A Unity sample project is available for download on this page, but we are not go
 Let's set up your Unity project to include the Social SDK package and add the necessary objects and scripts to use it.
 
 1. Create a new 2D project in Unity Hub using Unity version 2021.3 or later
-2. Open **Window > Package Manager**
-3. Click **+ > Add package from tarball...**
-4. In your project add a `Scripts` folder and create a `DiscordManager.cs` script
-5. Add the following code to `DiscordManager.cs`:
+2. Either:
+    1. Unzip the zip file in the `Packages` folder, or
+    2. Unzip the zip file to a temporary location, and [Install Package from Disk](https://docs.unity3d.com/Manual/upm-ui-local.html) from the temporary location.
+3. In your project add a `Scripts` folder and create a `DiscordManager.cs` script
+4. Add the following code to `DiscordManager.cs`:
 ```cs
 using UnityEngine;
 using UnityEngine.UI;