nitdoc: add a button to submit a bug

[privat]

When the doc is so bad and really fails to answer important questions, I want to complain now, because we are in 2016!

What about adding a link to the creation of an issue on github? Near the stars for instance.

Github allow to prefill some fields of a new issue
https://github.com/nitlang/nit/issues/new?title=foo&body=bar

[ventilooo]

In the same idea, there is also the option with yes or no with the question : Did this help you ?
It could gave some data to see which pages has been the most useful or the worst.

[privat]

@ventilooo there is a star system in the new doc: http://nitweb.nitlanguage.org/