Document our `containerd` future plans

[gcapizzi]

Before stopping developing Garden, we were plans to delegate more and more work to containerd, and to eventually get rid of our runc backend. We should document these plans, and their blockers.

• Remove the containerd without processes mode
  • this shouldn't have any blockers
• Replace grootfs with the snapshotter in full containerd mode
  • We couldn't find a way to implement the Volumizer interface using snapshotter, it felt like container creation and rootfs creation were tightly coupled.
  • We considered creating a new Gardener (that is, a new implementation of garden.Backend), fully backed by containerd.
  • This is probably impossible without refactoring the way peas are implemented. We want the peas flow to create containers using Gardener instead of depending directly on low-level implementation details.
• Remove runc mode
  • We have some users who have been unable to migrate to containerd due to issues with the shim freezing (see Garden failing with error "open mnt namespace: No such file or directory" #154 and #176654373 on Tracker).