Mapping for MIDI Controller Behringer DDM4000

[git-developer]

Implementation PR: mixxxdj/mixxx#3542

[uklotzde]

@Holzhaus @Swiftb0y I don't have permission to merge the PR. Please do so. I also cannot approve the CI workflows.

[Holzhaus]

Link check failure is unrelated.

@git-developer Thanks for the documentation so far, but I think it could be a bit more detailed. Could you add more documentation for each button (e.g. like the Pioneer DDJ-200 documentation does?

Since the mapping was already merged, I'll merge the documentation now, but I'd appreciate if you could make a follow-up PR.

CC @uklotzde

[git-developer]

Thanks for the approval and merge. I'll have a look into the referenced documentation as suggested and add another documentation PR.