[Ada] convert GNAT doc to sphinx

[Joseph Myers]

On Sun, 22 Feb 2015, Arnaud Charlet wrote:

> 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

-- 
Joseph S. Myers
joseph@codesourcery.com