GCC documentation: porting to Sphinx

[Arnaud Charlet]

> >In particular can you explain the motivation behind all the changes in the
> >gcc/ada/doc directory?
> 
> Sure:
> 1) All Sphinx manuals live in a directory where index page is called index.rst. That's why
> I moved e.g. this: gcc/ada/doc/{gnat_rm.rst => gnat_rm/index.rst}
> 2) I moved latex_elements.py to ada_latex_elements.py as it clashes with Sphinx global variable
> you modify in Sphinx config files
> 3) I created a shared Ada config (adabaseconf.py) that extends doc/baseconf.py and sets what
> is shared in between 3 Ada manuals.
> 4) gnu_free_documentation_license.rst is taken from $root/doc/

OK, this is really lots of changes, if we could minimize these changes
that would be best (and sorry but posting a link to a tarball also doesn't
help reviews, it was actually better with a link to a git repo previously...
At least the Ada part itself shouldn't be too big in particular once
simplified so could be posted standalone).

Arno