Design Doc for Subscription feature backend

Issue Description

This issue is part of a plan to onboard new users in Real Dev Squad. We need to implement a subscription feature where users can enter their phone number and email ID to receive notifications. This feature will help us gauge the interest level of potential users and understand the volume of users we may need to handle in the future, particularly those interested in joining the RDS Discord server.
This can also be used for the current members too. They can use it for subscribing to upcoming events or maybe some other information.

Expected Behavior

Currently the plan is to create a design doc for this which includes all of the following points :

• Users should be able to enter their phone number and email ID in the subscription form on the website.
• The form should validate the inputs to ensure that only valid phone numbers and email addresses are accepted.
• Upon successful form submission, the user's phone number and email ID should be stored securely in the database.
• Users should receive a confirmation email acknowledging their subscription.
• The system should track and display the number of subscriptions to provide insights into user interest.

Current Behavior

Nothing happens as of now.

Severity/Priority

• Critical
• High
• Medium
• Low