Firebase hosting for docs (#4871)

* configs for firebase hosting

* use the implicit FIREBASE_TOKEN

* don't print every command (lowers the chance of printing secret env vars, and for realz only deploy when merging into master

dev/docs/firebase.json

@@ -0,0 +1,11 @@
+{
+  "rules": "firebase_rules.json",
+  "hosting": {
+    "public": "doc",
+    "ignore": [
+      "firebase.json",
+      "**/.*",
+      "**/node_modules/**"
+    ]
+  }
+}