Update codespaces setup in get-started

[rfay]

Codespaces setup in get-started should be updated to match https://ddev.readthedocs.io/en/latest/users/install/ddev-installation/#ddev-installation-codespaces

We need to add the post-create command for it to work, due to a bug in codespaces:

{
  "image": "mcr.microsoft.com/devcontainers/universal:2",
  "features": {
    "ghcr.io/ddev/ddev/install-ddev:latest": {}
  },
  "postCreateCommand": "echo 'it should all be set up'"
}