Add migration guide Spock 1.x → 2.x to manual or separate document

Somewhere in another issue here (I don't remember where) we lately also talked about how I had difficulties migrating from Spock 1.x to Spock 2.x regarding my Maven setup. @Vampire held the opinion that this is all easily done and well explained somewhere in sample projects, but I had problems anyway and I am not the only one, see for instance this question which I have answered today because I learned by doing it already and had Björn to help me when I got stuck at some point, trying to get PowerMock running again.

The Spock manual currently just points to the JUnit manual for users to learn how to configure JUnit 5:
http://spockframework.org/spock/docs/2.0-M2/all_in_one.html#_new_junit_platform

But JUnit 5 is not Spock 2.x yet. Furthermore, many users are not just migrating from Spock 1.x to 2.x but also have existing JUnit 4 or 5 tests. So how do I set up my Maven project in order to be able to run either Junit 4 + Spock or JUnit 5 + Spock tests? These are real world problems, and an explanation in the manual could ease the pain of many users, I suppose.

Start by reading the manual's _new_junit_platform section and reviewing the linked migration discussion and Maven setup question. Document the Spock 1.x to 2.x migration path, including projects combining Spock with JUnit 4 or JUnit 5. Done means the manual or a linked standalone guide clearly covers these real-world configurations.