Mention example instance in api documentation

[Kaligule]

Issue details

When looking at the api documentation, I am asked to install an instance of wallabag and the reffere to the documentation at http://localhost:8000/api/doc

This is great when you want to test your code, but amying when you just have to look up some api call.

I think we should mention the instance at http://v2.wallabag.org, so everyone in a hurry can look at http://v2.wallabag.org/api/doc