Create release-aligned milestones and assign open issues

[DocGerd]

No milestones exist. Add 4 buckets mirroring gitflow and roll them at each release cut alongside the CHANGELOG ritual (#131/#132).

Acceptance criteria

• Create milestones v0.4.0, v0.5.0, Backlog, Icebox.
• Assign every open issue exactly one milestone.
• Add "roll milestones forward" to the release-cut docs sweep (Release-cut documentation sweep (do with the next develop→main release) #132).