Initializes CI CD setup

[suryabulusu]

• Adds prod.yaml and staging.yaml templates for lambda function initialization
• Adds github workflows for staging and production (via aws sam deploy)
• Adds Mangum adapter in main.py so that handler can run via lambda function
• Updates readme
• Production deployment is triggered when release branch is updated. Staging deployment is triggered when any push is sent.

(already added secrets for each environment -- as of now, they are the same)