Create devonfw-ide to IDEasy settings migration guide

[jan-vcapgemini]

We need a documentation for legacy devonfw-ide users which describes how their old settings need to be adjusted for our new IDEasy settings.

[hohwille]

See also #820

[julia-cap]

@hohwille, is this ticket obsolete due to having the auto-upgrader now or should the guide still be written?

[hohwille]

IMHO done. If you are missing something, please create a dedicated issue.