This patch makes it easier to find the right document describing image
requirements for Linux VM images. It does not link the RHEL page
specifically because it is mainly for RHEL 6 and 7 whereas we build RHEL
8. It instead links documentation for generic Linux distribution.
This is the start to document the image types that osbuild-composer
produces on a higher level than the code in the `distro` package.
The idea is to document the purpose for each of the image types as well
as the reasons why deviating from a standard install is necessary.
