Document CUDA & Java versions used in a toolchain generation #344

Contributors to EasyBuild might not be aware of the used versions for CUDA & Java in a toolchain generation.
While one can easily search for the CUDA version, as this is most likely included in the versionsuffix, checking for Java is more complicated.

We should add some documentation for this, e.g. in common-toolchains.md to mention the preferred Java & CUDA versions.

In addition, we should add some documentation pointing users towards LTS Java releases, as those are supported for a longer time.