Consider replacing FIXME.ORG with EXAMPLE.ORG in default configuration and docs

CiviCRM's default configuration has historically used FIXME.ORG as the domain, but FIXME.ORG is a valid domain which is not registered to the CiviCRM project.

If someone configured FIXME.ORG MX, they would likely capture bounces for sites which have not been correctly configured.

People should configure their sites correctly too of course, but using RFC2606 domains would prevent this method of incorrect configurations leaking CiviCRM data.