Avoid links to specific versions in documentation

[RvanVeenendaal]

In the documentation on https://github.com/openpreserve/ddhn-forge#downloading-the-virtual-machine there are links to specific versions of the VRE. Now that there are newer versions of the OVA (and VirtualBox 7), people following these links download an outdated version. Let's check the documentation and update all links to the VRE download folder and use language like "the latest version of the VRE", "the latest version of VirtualBox and accompanying extension pack", etc. The Dutch DDHN online guide will be updated accordingly.