docs: mention .env (#1463)

* Update getting-started.md

fix for 

The default Firebase app does not exist. Make sure you call initializeApp() before using any of the Firebase services.

* Update docs/nuxt/getting-started.md [skip ci]

---------

Co-authored-by: Eduardo San Martin Morote <[email protected]>

Author: subinznz

docs/nuxt/getting-started.md

@@ -65,7 +65,7 @@ export default defineNuxtConfig({
 
 ### Configuring the Admin SDK
 
-If you are using SSR with any auth related feature, you will need to create a [service account](https://firebase.google.com/support/guides/service-accounts) and provide its content as an _environment variable_ named `GOOGLE_APPLICATION_CREDENTIALS`.
+If you are using SSR with any auth related feature, you will need to create a [service account](https://firebase.google.com/support/guides/service-accounts) and provide its content as an _environment variable_ named `GOOGLE_APPLICATION_CREDENTIALS` in the `.env` file. 
 
 In local development it's more convenient to put the `service-account.json` file alongside other files and refer to its path in the environment variable: