Add `.env.example` for Environment Variables

[antoinekm]

Hi @harlan-zw,
It took significant time to locate and identify each required environment variable, such as NUXT_POSTMARK_API_KEY. To streamline the process for other developers, we need a .env.example file that outlines all necessary environment variables. This will save time and reduce confusion during setup.

Tasks:

1. Add a .env.example file to the project root.
2. Include placeholders for all required environment variables.
3. Add comments to explain the purpose of each variable.