Custom actions and verify `env` field is not in skaffold.dev yaml reference page

### Expected behavior
Be able to see the docs for the supported `env` field in https://skaffold.dev/docs/references/yaml/#customActions and https://skaffold.dev/docs/references/yaml/#verify

### Actual behavior
There is no reference for the `env` field in https://skaffold.dev/docs/references/yaml/#customActions or https://skaffold.dev/docs/references/yaml/#verify documentation pages.

E.g,

Per the examples in the integration tests folder, this field is supported:
- [Custom actions example](https://github.com/GoogleContainerTools/skaffold/blob/f9bdcce80ac184e29117353328c0de95ecd41c0a/integration/testdata/custom-actions-k8s/skaffold.yaml#L19)
- Verify follows the same structure