add cc0 license to site.cfg

Signed-off-by: Leah Rowe <info@minifree.org>

site.cfg

+# SPDX-License-Identifier: CC0-1.0
 TITLE="Canoeboot"
 DOMAIN="https://canoeboot.org/"
 BLOGDIR="news/" # leave as empty string if you want the blog to be the homepage