Start enabling substitutes from bordeaux.guix.gnu.org.

Start enabling substitutes from bordeaux.guix.gnu.org.

In addition to substitutes from ci.guix.gnu.org.  There are more changes that
can be made in the future, but these changes seem like a good start.

* config-daemon.ac (guix_substitute_urls): Add https://bordeaux.guix.gnu.org.
* guix/scripts/substitute.scm (%default-substitute-urls): Add
http://bordeaux.guix.gnu.org.
* guix/store.scm (%default-substitute-urls): Add bordeaux.guix.gnu.org.
* doc/guix.texi: Adjust accordingly.
* doc/contributing.texi: Adjust accordingly.