[Ada] convert GNAT doc to sphinx

[Arnaud Charlet]

> > Two .png files were missing, now added:
> > 
> > 2015-02-22  Arnaud Charlet  <charlet@adacore.com>
> >                       
> > 	* doc/gnat_ugn/project-manager-figure.png,
> > 	doc/gnat_ugn/rtlibrary-structure.png: New.     
> 
> The maintainer-scripts/update_web_docs_svn script is still producing
> errors relating to those files: 
> https://gcc.gnu.org/ml/gccadmin/2015-q1/msg00129.html

Indeed. I'll need some help here since I do not know where these .png files
are looked for or how to change that (I tried copying the .png file under
gcc/ada but this didn't help. I also tried adding -I$(srcdir)/ada/doc/gnat_ugn
to the doc/gnat_ugn.dvi rule in gcc/ada/gcc-interface/Make-lang.in rule, with
no success).

Basically the images can be found under gcc/ada/doc/gnat_ugn so for someone
familiar with texinfo this should be an easy change.

Arno