I would like to appoint a new maintainer for my package and could not find any documentation on this topic. What do I have to do to hand over my package to a new maintainer?

Have you already identified a new maintainer, or are you looking to find one?

If you're looking for a new maintainer, then advertising here and especially on the bioc-devel mailing list would be appropriate, but probably the most success will come from a 'power user' that you've interacted with already.

Once a maintainer is identified, send a request for maintainer responsibilities to be transferred to packages@bioconductor.org and be sure that both the new and old maintainers on the email.

Since this question is about package development rather than package use, it should really have been posted to the bioc-devel mailing list.