[Ada] convert GNAT doc to sphinx

[Arnaud Charlet <charlet@adacore.com>]

As discussed last year, we've converted the GNAT main documentation
(gnat_rm.texi and gnat_ugn.texi) to reST/sphinx, so the master doc
can now be found under gcc/ada/doc.

We're keeping automatically generated .texi files for now under gcc/ada
so that people only having texinfo tools can still generate the documentation
in various formats.

Once GCC has more generally switched to reST/sphinx, we can finalize the
transition and make the .rst files first class citizens/built by default.

Currently you need to manually call the Makefile in the doc subdirectory
to generate documentation using sphinx, e.g:

make -C doc
will yield a help on the various available build targets:
<<
Please use `make <target>' where <target> is one of
  DOC_NAME.html       to make standalone HTML files
  DOC_NAME.pdf        to make LaTeX files and run them through pdflatex
  DOC_NAME.txt        to make text files
  DOC_NAME.texinfo    to make Texinfo files
  DOC_NAME.info       to make info files
  DOC_NAME.all        to build DOC_NAME for all previous formats
  all                 to build all documentations in all formats
  html-all            same as previous rule but only for HTML format
  pdf-all             same as previous rule but only for PDF format
  txt-all             same as previous rule but only for text format
  texinfo-all         same as previous rule but only for texinfo format
  info-all            same as previous rule but only for info format

DOC_NAME should be a documentation name in the following list:
  gnat_rm gnat_ugn

source and location can be overriden using SOURCEDIR and BUILDDIR variables
>>

For example:

$ make -C doc all
$ make -C doc gnat_ugn.pdf

The SOURCEDIR and BUILDDIR variables have been introduced in
doc/Makefile in order to ease further integration with the GCC Makefiles.

Not including the diff which is very large and not very interesting.

Tested on x86_64-pc-linux-gnu, committed on trunk.

2015-02-20  Arnaud Charlet  <charlet@adacore.com>                      

        * gnat_rm.texi, gnat_ugn.texi: Now automatically generated from
        sphinx in the doc directory.
        * doc: New directory containing sphinx versions of gnat_rm and gnat_ugn