doc: relocate xserver.ent in the package root directory

This is the appropriate location for reusable configuration and/or
makefile artifacts as opposed to picking one of the subdirs where
it is used.

It shields them from future doc reorg as every subdir
will refer to the root package which never changes location.

Reviewed-by Jeremy Huddleston <jeremyhu@apple.com>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>
Signed-off-by: Keith Packard <keithp@keithp.com>