Refactor: Update NotFound page styling

[teleaziz]

Purpose

The goal of this pull request is to clean up and improve the styling of the 404 "Not Found" page. Based on the conversation history, the user wanted to refactor the styling and specifically remove the usage of h tag variants in the Typography components.

Code changes

• The h3 variant Typography component was replaced with two separate Typography components for "404" and "Page Not Found".
• Removed variant="h3" and component="h1" in favor of using the sx prop for custom styling, including fontWeight and responsive fontSize.
• Added padding and gap to the container Box for improved spacing.
• Styled the descriptive text Typography component by setting color, maxWidth, and textAlign for better visual presentation.

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