nginx: use /etc/nginx/nginx.conf enabling conf.d/

nginx: use /etc/nginx/nginx.conf enabling conf.d/

Instead of the default nginx.conf file this file is a small variant
without examples that enables the /etc/nginx/conf.d/ directory.

It will pull in all configuration files from the conf.d directory.
So, other packages can add their server parts in the conf.d directory
without modifying the main nginx.conf file (cf. #9860).

Changed also the default logging behavior:
	error_log stderr; # the init forwards it to logd
	access_log off;

See the updated documentation at:
https://openwrt.org/docs/guide-user/services/webserver/nginx

Signed-off-by: Peter Stadler <redacted>