Update docs for 1.6 release (#6093)
Update docs for 1.6 release (#6093)

Summary: version and links updates

JIRA issues: MARATHON-8132
gsantovena/marathon,mesosphere/marathon,mesosphere/marathon,guenter/marathon,meln1k/marathon,gsantovena/marathon,meln1k/marathon,guenter/marathon,guenter/marathon,meln1k/marathon,gsantovena/marathon,mesosphere/marathon,guenter/marathon,meln1k/marathon,mesosphere/marathon,mesosphere/marathon,gsantovena/marathon,meln1k/marathon,guenter/marathon,gsantovena/marathon