Merge: Phasify nitdoc

Launch nitdoc as a global phase (like metrics or nitg)

First commit is a cleanup that really detach doc/* from the modelbuilder.

Then some cleanup on `is_fictive`

Following commit remove the class NitdocContext and use ToolContext instead.

Last commit create add NitdocPhase that just instantiate and run Nitdoc.

Pull-Request: #587
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>