doc: Improve the first chapter text and definitions

In the beginning of the chapter, it was defined what is the so called "X"
thing and was removed the "Wayland" and "Weston" definitions cause we're
defining later at 1.2 anyway.

"Introduction", "Motivation" and "Compositing manager as the display server"
names sound better a bit than "Overview", "Replacing X11" and "Make the
compositing manager the display server" respectively. That was changed also.

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>