Mark 6.0.0-alpha1 as prerelease
release-state can be: released | prerelease | unreleased
//////////
-:release-state: unreleased
+:release-state: prerelease
:ref: https://www.elastic.co/guide/en/elasticsearch/reference/{branch}
:defguide: https://www.elastic.co/guide/en/elasticsearch/guide/master
