[Enhancement]: Add Success Message to Newsletter Form

[akshay0611]

Problem:
The current newsletter form lacks feedback for users after submission, which may lead to confusion about whether their subscription was successful.

Proposed Solution:

• Add a success message that appears after the form is submitted successfully.
• Use JavaScript to handle the form submission and dynamically display the success message.

Expected Outcome:
A newsletter form that provides clear feedback to users after submission, improving user experience and engagement.