Update the User's guide with the changes done to naming in the 2.0.0 release

[dietmarw]

There have been major updates to the naming conventions in the 2.0.0 release of OpenHPL. As usual, there was not enough time to also recreate the User's Guide document. So this is a leftover from #9.

One idea would actually be to convert the User's Guide from an external document to add it to the internal ´OpenHPL.UsersGuide`.