Add documentation explaining the config files

[M-Davies]

We have recently added a lot of new attributes/features to our jdkxx_pipeline_config.groovy files (#2100, #2051, #1952, etc.). Yet, we have no documentation explaining what attributes there are in the config files. We would like a piece of documentation explaining what the config files do and how they can be modified.

[M-Davies]

Dup of #2129 (this was made first but the other issue explains the problem better so we'll roll with that one) 🙂