feat: Add success message to newsletter form

[akshay0611]

Summary: This PR adds a success message to the newsletter form to improve user feedback. After a user submits the form, a success message is displayed to confirm their subscription.

Changes Made:

• Added a hidden success message element (<p class="success-message">) below the form.
• Included JavaScript to handle form submission and display the success message dynamically.
• Ensured the input field is cleared after submission for a clean user experience.

Testing:

• Verified that the success message appears after form submission.
• Tested responsiveness across different screen sizes.
• Ensured alignment with existing CSS styles and framework conventions.

Related Issue: Closes #7