docs(getting-started): configuration page (#7689)

Co-authored-by: Sébastien Chopin <seb@nuxtjs.com>
Co-authored-by: Daniel Roe <daniel@roe.dev>
Co-authored-by: Pooya Parsa <pooya@pi0.io>

docs/content/1.getting-started/3.configuration.md

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+# Configuration
+
+By default, Nuxt is configured to cover most use cases. The [`nuxt.config.ts`](/guide/directory-structure/nuxt.config) file can override or extend this default configuration.
+
+## Nuxt Configuration
+
+The `nuxt.config.ts` file is located at the root of a Nuxt project and can override or extend the application's behavior.
+
+A minimal configuration file exports the `defineNuxtConfig` function containing an object with your configuration. The `defineNuxtConfig` helper is globally available without import.
+
+```ts [nuxt.config.ts]
+export default defineNuxtConfig({
+  // My Nuxt config
+})
+```
+
+This file will often be mentioned in the documentation, for example to add custom scripts, register modules or change rendering modes.
+
+::alert{type=info}
+Every configuration option is described in the [Configuration Reference](/api/configuration/nuxt.config).
+::
+
+::alert{type=info}
+You don't have to use TypeScript to build an application with Nuxt. However, it is strongly recommended to use the `.ts` extension for the `nuxt.config` file. This way you can benefit from hints in your IDE to avoid typos and mistakes while editing your configuration.
+::
+
+### Environment Variables and Private Tokens
+
+The `runtimeConfig` API exposes values like environment variables to the rest of your application. By default, these keys are only available server-side. The keys within `runtimeConfig.public` are also available client-side.
+
+Those values should be defined in `nuxt.config` and can be overridden using environment variables.
+
+::code-group
+
+```ts [nuxt.config.ts]
+export default defineNuxtConfig({
+  runtimeConfig: {
+    // The private keys which are only available server-side
+    apiSecret: '123',
+    // Keys within public are also exposed client-side
+    public: {
+      apiBase: '/api'
+    }
+  }
+})
+```
+
+```text [.env]
+# This will override the value of apiSecret
+NUXT_API_SECRET=api_secret_token
+```
+
+::
+
+These variables are exposed to the rest of your application using the [`useRuntimeConfig`](/api/composables/use-runtime-config) composable.
+
+```vue [pages/index.vue]
+<script setup>
+const runtimeConfig = useRuntimeConfig()
+</script>
+```
+
+:ReadMore{link="/guide/going-further/runtime-config"}
+
+## App Configuration
+
+The `app.config.ts` file, also located at the root of a Nuxt project, is used to expose public variables that can be determined at build time. Contrary to the `runtimeConfig` option, these can not be overriden using environment variables.
+
+A minimal configuration file exports the `defineAppConfig` function containing an object with your configuration. The `defineAppConfig` helper is globally available without import.
+
+```ts [app.config.ts]
+export default defineAppConfig({
+  title: 'Hello Nuxt',
+  theme: {
+    dark: true,
+    colors: {
+      primary: '#ff0000'
+    }
+  }
+})
+```
+
+These variables are exposed to the rest of your application using the [`useAppConfig`](/api/composables/use-app-config) composable.
+
+```vue [pages/index.vue]
+<script setup>
+const appConfig = useAppConfig()
+</script>
+```
+
+:ReadMore{link="/guide/directory-structure/app.config"}
+
+## `runtimeConfig` vs `app.config`
+
+As stated above, `runtimeConfig` and `app.config` are both used to expose variables to the rest of your application. To determine whether you should use one or the other, here are some guidelines:
+
+- `runtimeConfig`: Private or public tokens that need to be specified after build using environment variables.
+- `app.config` : Public tokens that are determined at build time, website configuration such as theme variant, title and any project config that are not sensitive.
+
+Feature                        | `runtimeConfig`  | `app.config`
+-------------------------------|------------------|-------------------
+Client Side                    | Hydrated         | Bundled
+Environment Variables          | ✅ Yes           | ❌ No
+Reactive                       | ✅ Yes           | ✅ Yes
+Types support                  | ✅ Partial       | ✅ Yes
+Configuration per Request      | ❌ No            | ✅ Yes
+Hot Module Replacement         | ❌ No            | ✅ Yes
+Non primitive JS types         | ❌ No            | ✅ Yes