feat(site): add dotenv section

.internal/site/.vitepress/config.mts

@@ -46,6 +46,10 @@ export default defineConfig({
             text: "Routing",
             link: "/introduction/routing",
           },
+          {
+            text: "Dotenv",
+            link: "/introduction/dotenv",
+          },
         ],
       },
       {

.internal/site/src/introduction/dotenv.md

@@ -0,0 +1,17 @@
+# Dotenv
+
+Dotenv is the de facto standard, but it lacks validation, making it prone to errors. In this template, we use zod to validate the required environment variables in files like `next.config.ts`. If any required variables are missing or invalid, the application will fail to execute. This approach ensures robustness at runtime.
+
+::: code-group
+
+<<< ../../../../.env.sample
+
+<<< ../../../../env.ts
+
+:::
+
+If you need to add new environment variables, make sure to add them to both `.env` and `.env.test`, and update the `env.ts` with the corresponding zod validation.
+
+## Why not use .env.local?
+
+Prisma can read the `.env` file, and the `DATABASE_URL` is a required key for migration. While Next.js prioritizes `.env.local`, splitting the dotenv files can be inefficient. Therefore, to maintain consistency with Prisma, this template uses `.env`.