Revise Docs Maintainers list

Removing Sonat from docs maintainers because he no longer has time to complete all the responsibilities. Many thanks to Sonat for all of his help.

Pinging @shykes @SvenDowideit @jamtur01 

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)