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)