[Bug] Please document the sandbox url as a new feature

[CBoensel]

Library used

both paypal-js and react-paypal-js

🐞 Describe the Bug

sandbox is supported. see https://github.com/paypal/paypal-js/blob/main/packages/paypal-js/src/utils.ts#L73

I do not see it documented though.

😕 Actual Behavior

No indication of how to enter sandbox mode.

🤔 Expected Behavior

Documentation of the parameter environment and it's allowed values such as "sandbox".

• Add param to the read me markdown files of each component.
• File a ticket to the technical writers to add it on https://developer.paypal.com/sdk/js/configuration/ as well.