Change vendor.js to app.js in documentation

[bendemboski]

Because of how blunt broccoli-asset-rev is, the occurences of vendor.js in our documentation get rewritten with the fingerprinted filename vendor-<hash>.js, which is weird and confusing in the generated documentation site. So change to app.js which doesn't match any assets and won't get rewritten (and it just as good for the documentation/example purposes).