Is there any web API that takes a URL and gives back the readable content of that page? Or is there any javascript library that can do this?
Installing node.js or any server-side app is not possible.
That's actually a really hard problem. Such questions usually imply that you want sidebars and similar 'noise' elements excluded. This is often solved with different training sets within the machine learning computer domain and it's not easy to develop.
Having said that - take a look at:
This should help you to learn enough about the space that you'll be able to do better searches for you problem domain.