build(aio): prevent Windows error on serve-and-sync (angular#21806)

Running `yarn start` (which watches the `src/` directory) and
`yarn docs-watch` (which cleans up files in `src/generated/api/`) often
results in `ENOTEMPTY` errors.

This commit solves it by ensuring that `yarn docs` has been completed
before running `yarn start`.

PR Close angular#21806

Author: gkalpak

aio/package.json

@@ -44,7 +44,8 @@
     "docs-lint": "eslint --ignore-path=\"tools/transforms/.eslintignore\" tools/transforms",
     "docs-test": "node tools/transforms/test.js",
     "tools-test": "./scripts/deploy-to-firebase.test.sh && yarn docs-test && yarn boilerplate:test && jasmine tools/ng-packages-installer/index.spec.js",
-    "serve-and-sync": "concurrently --kill-others \"yarn docs-watch\" \"yarn start\"",
+    "preserve-and-sync": "yarn docs",
+    "serve-and-sync": "concurrently --kill-others \"yarn docs-watch --watch-only\" \"yarn start\"",
     "boilerplate:add": "node ./tools/examples/example-boilerplate add",
     "boilerplate:remove": "node ./tools/examples/example-boilerplate remove",
     "boilerplate:test": "node tools/examples/test.js",