Does my "index.html" also serve as a static resource?
It depends if you declare it a static resource or not.
You can declare resources as static via appengine-web.xml (look for <public-root> and <static-files>). Static resources are served directly by GAE, without chewing into your app's instance hours quota.
Note: if your app only serves static content you might find it more convenient to use the app.yaml config file instead of appengine-web.xml (as non-Java apps do) - the actual app sandbox doesn't really matter since you don't actually have app code in this case. See Serving Static Files and/or look for static_dir and static_files in the app.yaml Reference.
