docs(config): add runtime config and environment variables section

docs/content/1.guide/1.configuration.md

@@ -37,3 +37,84 @@ timing=true
 nitro.timing=true
 ```
 ::
+
+## Runtime Configuration
+
+Nitro provides a runtime config API to expose configuration within your application, with the ability to update it at runtime by setting environment variables.
+
+To expose config and environment variables to the rest of your app, you will need to define runtime configuration in your configuration file.
+
+**Example:**
+
+::code-group
+```ts [nitro.config.ts]
+export default defineNitroConfig({
+  runtimeConfig: {
+    helloThere: "foobar",
+  }
+})
+```
+
+```ts [nuxt.config.ts]
+export default defineNuxtConfig({
+  runtimeConfig: {
+    helloThere: "foobar",
+  }
+})
+```
+::
+
+You can now access the runtime config using `useRuntimeConfig(event)`.
+
+**Example:**
+
+::code-group
+```ts [api/example.get.ts (nitro)]
+export default defineEventHandler((event) => {
+  return useRuntimeConfig(event).helloThere // foobar
+});
+```
+
+```ts [server/api/example.get.ts (nuxt)]
+export default defineEventHandler((event) => {
+  return useRuntimeConfig(event).helloThere // foobar
+});
+```
+::
+
+::alert{type="info"}
+**Note:** Consider using `useRuntimeConfig(event)` within event handlers and utilities and avoid calling it in ambient global contexts.
+::
+
+
+### Environment Variables
+
+Nitro supports defining environment variables using `.env` file in development (use platform variables for production).
+
+Create an `.env` file in your project root:
+
+```bash [.env]
+TEST="123"
+```
+
+You can universally access environment variables using `import.meta.env.TEST` or `process.env.TEST`.
+
+::alert{type="info"}
+**Note:** Consider writing any logic that depends on environment variables within event handlers and utilities and avoid accessing and caching them in ambient global contexts.
+::
+
+### Update runtime config using environment variables
+
+The variables prefixed with `NITRO_` will be applied to runtime config, and they will override the variables defined within your `nitro.config.ts` file. (matching "camelCase" version).
+
+**Example:**
+
+::code-group
+```bash [.env (nitro)]
+NITRO_HELLO_THERE="123"
+```
+
+```bash [.env (nuxt)]
+NUXT_HELLO_THERE="123"
+```
+::