docs: add Windows PowerShell equivalents for environments and scripting (#20333)

docs/cli/tutorials/mcp-setup.md

@@ -20,10 +20,18 @@ Most MCP servers require authentication. For GitHub, you need a PAT.
     **Read/Write** access to **Issues** and **Pull Requests**.
 3.  Store it in your environment:
 
+**macOS/Linux**
+
 ```bash
 export GITHUB_PERSONAL_ACCESS_TOKEN="github_pat_..."
 ```
 
+**Windows (PowerShell)**
+
+```powershell
+$env:GITHUB_PERSONAL_ACCESS_TOKEN="github_pat_..."
+```
+
 ## How to configure Gemini CLI
 
 You tell Gemini about new servers by editing your `settings.json`.