Add a Cloud Build config to deploy website to Firebase. (#5447)

As the title says. This is the Cloud Build configuration that we'll use
to build dart.dev and publish to Firebase hosting.

Note: This current config currently publishes to a staging Flutter
project to confirm everything works on main.

Once this PR lands and we've confirmed deploy works as expected, 
- [ ] I'll open another PR to switch to prod project `dart-dev`
- [ ] Decommission the [deploy.yaml GitHub
action](https://github.com/dart-lang/site-www/blob/main/.github/workflows/deploy.yml).
- [ ] Update the trigger branch to `main`

cc @drewroengoogle @parlough

cloud_build/deploy.yaml

@@ -0,0 +1,19 @@
+steps:
+  - name: gcr.io/cloud-builders/git
+    args: ['submodule', 'update', '--init', '--recursive']
+  - name: gcr.io/cloud-builders/docker
+    entrypoint: '/bin/bash'
+    args:
+      - '-c'
+      - |-
+        set -e
+        echo "Building the website using a makefile..."
+        make build
+  - name: gcr.io/flutter-dev-230821/firebase-ghcli
+    entrypoint: '/bin/bash'
+    args:
+      - '-c'
+      - |-
+        firebase deploy --project=flutter-website-staging-349021 --only=hosting
+options:
+  logging: CLOUD_LOGGING_ONLY