fix: removed additional YAML document header when rendering CRDs

[shruti2522]

What this PR does / why we need it:
Fixes #12953. I have modified the way CRD file data is rendered so that no additional headers are generated in the stdout.

Output before:

shruti@fedora:~/cncf/helm$ bin/helm template --include-crds cmd/helm/testdata/testcharts/subchart/ | head
---
# Source: subchart/crds/crdA.yaml
---
apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: testcrds.testcrdgroups.example.com
spec:
  group: testcrdgroups.example.com
  version: v1alpha1

Output after:

shruti@fedora:~/cncf/helm$ bin/helm template --include-crds cmd/helm/testdata/testcharts/subchart/ | head
---
# Source: subchart/crds/crdA.yaml

apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: testcrds.testcrdgroups.example.com
spec:
  group: testcrdgroups.example.com
  version: v1alpha1

[gjenkins8]

Thanks for the PR!

Can we include a test please (to prevent regression of this behavior in the future). Otherwise I agree with the approch in this PR -- the existing separator in the file needs to be removed if it exists, as Helm is adding its own (and if Helm didn't add it's own, yaml files without seperators would be rendered to stdout without yaml seperators, which also breaks other tools)

For additional context, I'm sure there was an issue/PR recently which fixed that last point. But I don't immediately see it in the git history.