Add .env.example for nextjs env variables

[Pabl0cks]

Description

This PR adds .env.example file for nextjs environment variables, like we already have for hardhat.

Also adds some extra information on how we are managing API Keys in config/env files in both hardhat and nextjs .env.example files, so users can have a better understanding.

Additional Information

• This is not a duplicate of any existing pull request

Your ENS/address: pabl0cks.eth

[carletex]

Great job @Pabl0cks !!

I pushed some little tweaks.

Also noticed that we use NEXT_PUBLIC_DEPLOY_BLOCK to improve performance in the useScaffoldEventHistory hook. I feel that we could move that env var from ContractData.tsx to the hook itself. Like use fromBlock if passed, if not use the env var, if not use 0. But we can discuss that in another issue. cc @technophile-04 @rin-st

[Pabl0cks]

Also noticed that we use NEXT_PUBLIC_DEPLOY_BLOCK to improve performance in the useScaffoldEventHistory hook. I feel that we could move that env var from ContractData.tsx to the hook itself. Like use fromBlock if passed, if not use the env var, if not use 0.

I think it's a great idea! So you don't have to manually set the fromBlock or add the process.env.NEXT_PUBLIC_DEPLOY_BLOCK ? logic in every useScaffoldEventHistory you use.