index: rewrite the introduction

This was inspsired by a recent IRC discussion where I once again
explained what Wayland is, and triggered by the IRC comment
< MrCooper> heh, https://wayland.freedesktop.org/

 says "GNOME and KDE are
                     expected to be ported to it."
< MrCooper> should be any day now

I tried to make the explanation more understandable, including what
Wayland is and what it is not.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>