ci(aio): fix deploying the stable branch to Firebase

The `deploy-to-firebase.sh` always expects there to be a
`src/extra-files/<mode>` directory and breaks if it doesn't exist.

aio/src/extra-files/README.md

@@ -6,4 +6,8 @@ After the AIO application had been built and before it is deployed all files and
 inside the folder with the same name as the current deployment mode (next, stable, archive)
 will be copied to the `dist` folder.
 
-See the `scripts/deploy-to-firebase.sh` script for more detail.
+See the `scripts/deploy-to-firebase.sh` script for more detail.
+
+**Note:**
+The `deploy-to-firebase.sh` script always expects there to be a folder for the current deployment
+mode (even if it is empty).

aio/src/extra-files/archive/robots.txt

@@ -1,2 +1,3 @@
+# Disallow all URLs (see http://www.robotstxt.org/robotstxt.html)
 User-agent: *
-Disallow: /
+Disallow: /

aio/src/extra-files/next/robots.txt

@@ -1,2 +1,3 @@
+# Disallow all URLs (see http://www.robotstxt.org/robotstxt.html)
 User-agent: *
-Disallow: /
+Disallow: /

aio/src/extra-files/stable/robots.txt

@@ -0,0 +1,3 @@
+# Allow all URLs (see http://www.robotstxt.org/robotstxt.html)
+User-agent: *
+Disallow: