Document skaffold command init behavior

## Background
- #7515 changed init cmd behavior to generate skaffold.yaml fits v3alphal1 schema
- If kustomize, raw manifests detected or cli provide manifests, generated skaffold config will have render config without deploy config
- If helm charts are detected, generated skaffold ocnfig will have deploy config for helm without render config
- We wants to document these behaviors on skaffold.dev

## Related Future work
- may add flags to customize these behaviors (needs discussions)