Update Github navbar link with icon

[MathyouMB]

As a visitor, I want the "GitHub" link in the navbar to use the GitHub icon instead of plain text so that it’s easier to recognize and visually consistent with other Docusaurus sites.

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🧠 Context

Right now, the top navigation bar includes a plain text link labeled "GitHub". Instead, we want to display the GitHub icon, as shown on the [official Docusaurus site](https://docusaurus.io/docs/cli).

✅ Example (what we want):

🔗 Current Icon (from Docusaurus docs):

This appears to be a built-in feature of Docusaurus — it’s likely just a matter of updating the navbar configuration in docusaurus.config.ts.

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🛠️ Implementation Plan

1. Open docusaurus.config.ts

2. Update the navbar item for GitHub

   • Replace the current text-based item with one that uses the built-in icon

   Example from the Docusaurus site:

   navbar: {
     items: [
       {
         href: 'https://github.com/CarletonComputerScienceSociety/handbook',
         position: 'right',
         className: 'header-github-link',
         'aria-label': 'GitHub repository',
       },
     ]
   }

3. Ensure icon CSS is included

   • Docusaurus automatically provides the icon if you use the header-github-link class

4. Test the navbar

   • Verify that the GitHub icon appears and links to the correct repository
   • Confirm it looks correct in both light and dark mode

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✅ Acceptance Criteria

• The "GitHub" text link is replaced with the GitHub icon
• The icon appears in the navbar and links to the correct repo
• Styling is consistent with other Docusaurus sites
• The icon works correctly in both light and dark themes