[wwwdocs] Import www.gnu.org style and start using it (fwd)

[Gerald Pfeifer]

I've been well aware of the fact that our web page is looking 90s-ish
and that www.gnu.org now has a new style, which looks much nicer than
when we created the current design for gcc.gnu.org, so we probably
should use it.

Import the CSS stylesheet from www.gnu.org and, initially, use it for
our obligatory footers.

I manually regenerated all page on gcc.gnu.org.

More to come...

Gerald

PS: The gcc.gnu.org spamfilters do not allow me to send the real patches,
but it's revision 1.84 -> 1.86 of style.mhtml and revision 1.1 of gnu.css.