legacy/migration: new custom description type for Notes

[slint]

I think we need to add a new custom description type (note), so that we can easily display this in the record landing page UI, in a similar way as we do currently on legacy (i.e. the yellow box above the files preview).

For now let's leave it as is, but once we address all the visual/UX issues we'll come across it.

Originally posted by @slint in https://github.com/zenodo/zenodo-rdm/pull/330#discussion_r1198959578