Adopt more than one pet

**Is your feature request related to a problem? Please describe.**
Our project currently does not allow users to adopt more than one pet at a time.

**Describe the solution you'd like**
We can implement a cart page where users can add two or more pets for adoption. Once a pet is added to the cart, users can proceed to checkout. After checkout, they will be directed to the adopt.html form page to fill in their details for registration.