web site changes

[Gaius Mulley]

Hello,

I've changed the web site so that by default the information relates to
the latest stable release.  I plan on having extra tabs for each
supported branch of gcc, under which will sit the older documentation.
I've also culled building info on other platforms for the recent
documentation and replaced it with a link to
https://gcc.gnu.org/wiki/InstallingGCC based on the premise that gm2 now
only uses libraries already used by gcc.

The other platform info will remain for releases < gcc-10  (as gm2 uses
libraries not found in gcc for those releases - libpth).

hope this is useful

regards,
Gaius