docs: add example of runtime domain configuration to the guide

docs/content/2.guide/10.different-domains.md

@@ -109,3 +109,19 @@ export default defineNuxtConfig({
   // ...
 })
 ```
+
+With the above config, a build would have to be run for staging and production with different .env files that specify `DOMAIN_UK` and `DOMAIN_FR`.
+
+Alternatively, to avoid the need for multiple builds, the locale domains can be overridden via runtime environment variables. The variable name should follow the format `NUXT_PUBLIC_I18N_LOCALES_{locale code}_DOMAIN`
+
+For example:
+
+```env {}[production .env]
+NUXT_PUBLIC_I18N_LOCALES_UK_DOMAIN=uk.example.test
+NUXT_PUBLIC_I18N_LOCALES_FR_DOMAIN=fr.example.test
+```
+
+```env {}[staging .env]
+NUXT_PUBLIC_I18N_LOCALES_UK_DOMAIN=uk.staging.example.test
+NUXT_PUBLIC_I18N_LOCALES_FR_DOMAIN=fr.staging.example.test
+```