With 8.13 CR3 released we should be near the final state of the release. The 8.13.x branch has been created for any additional 8.13 changes. The master branch will be updated to contain the 9.0.x changes. Ideally we'll have out the first 9.0 alpha in about 3 weeks.
Also Kylin has done a great job getting the docs converted from confluence. Since it is a rather involved process we will just cut over to gitbook/asciidoc for 9.0 and not 8.13/8.12.x.