Add an extensive migration guide to the documentation and/or website to help users migrate from Search 5 to Search 6

The migration guide should mention in particular the fact that users can proceed step by step:

• they can use the compatibility layer as a first step (see HSEARCH-3282), migrate all non-compiling code to 6, and only then migrate the rest of the code to 6 by removing the compatibility layer.

• if is fixed, they can use the ORM 5 integration as a first step, fix everything until their application works again, and only then move to the ORM 6 integration.

The guide should also mention, for every feature (annotation, API, ...) of Search 5 that changed, what should be used instead in Search 6.