docs: updated nuxt bridge migration guide (#7775)

Co-authored-by: Sébastien Chopin <seb@nuxtjs.com>
Co-authored-by: Daniel Roe <daniel@roe.dev>

docs/content/bridge/1.overview.md

@@ -149,6 +149,36 @@ Overwriting options such as `"compilerOptions.paths"` with your own configuratio
 In case you need to extend options provided by `./.nuxt/tsconfig.json` further, you can use the `alias` property withing your `nuxt.config`. `nuxi` will pick them up and extend `./.nuxt/tsconfig.json` accordingly.
 ::
 
+## Update Runtime Config
+
+Nuxt 3 approaches runtime config differently than Nuxt 2, using a new combined `runtimeConfig` option.
+
+First, you'll need to combine your `publicRuntimeConfig` and `privateRuntimeConfig` properties into a new one called `runtimeConfig`, with the public config within a key called `public`.
+
+```diff
+// nuxt.config.js
+- privateRuntimeConfig: {
+-   apiKey: process.env.NUXT_API_KEY || 'super-secret-key'
+- },
+- publicRuntimeConfig: {
+-   websiteURL: 'https://public-data.com'
+- }
++ runtimeConfig: {
++   apiKey: process.env.NUXT_API_KEY || 'super-secret-key',
++   public: {
++     websiteURL: 'https://public-data.com'
++   }
++ }
+```
+
+This also means that when you need to access public runtime config, it's behind a property called `public`. If you use public runtime config, you'll need to update your code.
+
+```diff
+// MyWidget.vue
+- <div>Website: {{ $config.websiteURL }}</div>
++ <div>Website: {{ $config.public.websiteURL }}</div>
+```
+
 ## Migrate Composition API
 
 If you were using `@vue/composition-api` or `@nuxtjs/composition-api`, please read the [composition api migration guide](/bridge/bridge-composition-api).