feat: Add Success Message to Newsletter Form

[akshay0611]

Summary: This PR adds a success message to the newsletter form to improve user experience. After submitting their email, users will see a success button indicating that their subscription was successful.

Changes Made:

• Added a hidden success message (<div id="successMessage">) to the form.
• Used JavaScript to handle form submission, hide the original form elements, and display the success message.
• Styled the success button with a green background to indicate success.

Testing:

• Verified responsiveness across different screen sizes.
• Ensured proper rendering of the success message after form submission.

Related Issue: Closes #26