docs: update Learning Hub with recent Copilot CLI and app features

- Document GPT-6 Astra model support and vim mode in the CLI (v1.0.84)
- Document Windows 11 taskbar taskbarPresence opt-out setting
- Document GitHub Copilot app v1.1.15 features: /goal command for
  autopilot, PR image/video attachments, MCP server install via deep
  link, and expanded stacked pull request support
- Cross-link the new MCP deep-link install flow from the MCP servers guide

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/src/content/docs/learning-hub/understanding-mcp-servers.md

@@ -3,7 +3,7 @@ title: 'Understanding MCP Servers'
 description: 'Learn how Model Context Protocol servers extend GitHub Copilot with access to external tools, databases, and APIs.'
 authors:
   - GitHub Copilot Learning Hub Team
-lastUpdated: 2026-09-05
+lastUpdated: 2026-09-09
 estimatedReadingTime: '8 minutes'
 tags:
   - mcp
@@ -112,6 +112,8 @@ You can also install a specific server by name directly:
 
 This guided flow is the recommended way to add new MCP servers, especially for servers that require multiple configuration values.
 
+**Installing via deep link** *(GitHub Copilot app, v1.1.15+)*: In the GitHub Copilot app, you can install an MCP server by opening a deep link. Instead of manually filling out the add-server form, the link opens a prefilled review form so you can confirm the server's name, command, and configuration before it's added — useful for sharing a one-click setup link with teammates. See [Getting Started with the GitHub Copilot app](../github-copilot-app/) for more on the app's deep link support.
+
 ### Configuration Fields
 
 **command**: The executable to run the MCP server (e.g., `npx`, `python`, `docker`).