build(aio): delete content folder before doc-gen (#15414)

This should prevent false-positive e2e test runs where stale files
are left lying around.

aio/package.json

@@ -17,6 +17,7 @@
     "deploy-preview": "scripts/deploy-preview.sh",
     "deploy-staging": "firebase use staging --token \"$FIREBASE_TOKEN\" && yarn ~~deploy",
     "check-env": "node ../tools/check-environment.js",
+    "predocs": "rimraf src/content",
     "docs": "dgeni ./transforms/angular.io-package",
     "docs-test": "node ../dist/tools/cjs-jasmine/index-tools ../../transforms/**/*.spec.js",
     "~~update-webdriver": "webdriver-manager update --standalone false --gecko false",
@@ -65,6 +66,7 @@
     "karma-jasmine-html-reporter": "^0.2.2",
     "lodash": "^4.17.4",
     "protractor": "~5.1.0",
+    "rimraf": "^2.6.1",
     "ts-node": "~2.0.0",
     "tslint": "~4.4.2",
     "typescript": "2.1.6"

aio/yarn.lock

@@ -5324,6 +5324,12 @@ rimraf@2, rimraf@^2.2.8, rimraf@^2.3.3, rimraf@^2.4.3, rimraf@^2.4.4, rimraf@^2.
   dependencies:
     glob "^7.0.5"
 
+rimraf@^2.6.1:
+  version "2.6.1"
+  resolved "https://registry.yarnpkg.com/rimraf/-/rimraf-2.6.1.tgz#c2338ec643df7a1b7fe5c54fa86f57428a55f33d"
+  dependencies:
+    glob "^7.0.5"
+
 rimraf@~2.2.6:
   version "2.2.8"
   resolved "https://registry.yarnpkg.com/rimraf/-/rimraf-2.2.8.tgz#e439be2aaee327321952730f99a8929e4fc50582"