More manual

This improves the documentation of the other nit tools.

It could be better to read them on the http://nitlanguage.org/tools/ since this PR also enable the synchronization of the manpage on the website.

Documentation of `nit` is copied from http://nitlanguage.org/debugger/
Documentation of `nitdoc` is copied from https://github.com/privat/nit/blob/master/src/testing/README

Note: a lot of options are common to the tools because they share the same frontend.
Usually, to simplify the manpages, I only indicated the options specific to the tool.

I am not fond to copy-paste each common options among all the tools, especially when the tool has only one or two original option but more than 30 *inherited* options.
Maybe creating a `nitcli(7)` manpage that regroup common options and conventions among our tools. (à la `gitcli(7)`) ?