Officially publish the new site

Publishing the web site means creating a publish branch and adding the right magic instructions to .asf.yml etc. This will then publish the new site and disable old CMS.

Before we do that we should

1. Make sure all docs and release tools are updated for new site publishing instructions
2. Create a PR with latest changes in old CMS site since the export. This will be the changes done during 8.3.0 release and possibly some news entries related to security issues etc.

After publishing we should ask INFRA to make old site svn read-only (and perhaps do a commit that replaces svn content with a README.txt), so it is obvious for everyone that we have migrated.