from  Benoît Perrot  <benoit@lrde.epita.fr>
        More documentation.

        * doc/copying.texi,
        * doc/internal.texi,
        * doc/invoking.texi,
        * doc/language.texi:
        Remove files, move their content to...
        * doc/mipsy.texi:
        ... this file.
        Augment documentation on invocation.