=doc clean up what-is-akka.rst and switch to www.typesafe.com

the latter is necessary because of broken DNS requirements which make
apex domains brittle (since they must resolve to an A record with a
single IP)
Merge pull request #2067 from akka/wip-WhatIsAkka-∂π

=doc clean up what-is-akka.rst and switch to www.typesafe.com