Merge: Move back manual to the main repo

The manual is slowly bit rotting in the wiki (the disclaimer in http://nitlanguage.org/manual/ and the link to the very old issue  #761 is not a nice thing to have).

The idea is to move back the documentation in the main repository so that PR that enhance the language should also update the manual.
We could add checks to ensure that new AST element or annotations implies some update in the manual.

It is what is currently done with the manpages of the tools, and they are quite complete and up to date.

Pull-Request: #2762

diff --cc .gitlab-ci.yml
Simple merge