Add Discord link to navigation bar

[Mr-Technician]

This pull request adds a Discord link with an SVG icon to the navigation bar and updates the navigation bar styling to better support multiple right-aligned links. The main changes are grouped into navigation functionality and styling improvements:

Navigation functionality:

• Added a new Discord link to the navigation bar, using a custom SVG icon for the Discord logo. The link opens in a new tab and uses appropriate security attributes. (src/Navbar.js)
• Imported SvgIcon from Material UI to support rendering the custom Discord SVG icon. (src/Navbar.js)

Styling improvements:

• Updated the CSS selector for right-aligned navigation links to apply styles to the last two links instead of only the last one, ensuring both the GitHub and Discord icons are styled correctly. (src/Navbar.css)