Better skaffold help output

[dgageot]

skaffold help says:

A tool that facilitates continuous development for Kubernetes applications.

Usage:
  skaffold [command]

Available Commands:
  build       Builds the artifacts
  completion  Output skaffold shell completion for the given shell (bash or zsh)
  config      A set of commands for interacting with the skaffold config.
  delete      Delete the deployed resources
  deploy      Deploys the artifacts
  dev         Runs a pipeline file in development mode
  diagnose    Run a diagnostic on Skaffold
  fix         Converts old skaffold.yaml to newest schema version
  help        Help about any command
  init        Automatically generate skaffold configuration for deploying an application
  run         Runs a pipeline file
  version     Print the version information

Flags:
  -h, --help               help for skaffold
  -v, --verbosity string   Log level (debug, info, warn, error, fatal, panic) (default "warning")

Use "skaffold [command] --help" for more information about a command.```

Our doc says:

End-to-end pipelines:
 + skaffold run - to build & deploy once
 + skaffold dev - to trigger the watch loop build & deploy workflow with cleanup on exit

Pipeline building blocks for CI/CD:
 + skaffold build - to just build and tag your image(s)
 + skaffold deploy - to deploy the given image(s)
 + skaffold delete - to cleanup the deployed artifacts

Getting started with a new project:
 + skaffold init - to bootstrap skaffold.yaml
 + skaffold fix - to upgrade from

Utilities:
 + skaffold help - print help
 + skaffold version - get Skaffold version
 + skaffold completion - setup tab completion for the CLI
 + skaffold config - manage context specific parameters
 + skaffold diagnose - diagnostics of skaffold works in your project

I think it would be nice to organize the output of skaffold help like our documentation.
By default, cobra can't manage command groups but it should be possible to twist his arm like kubectl does.

[dgageot]

This was fixed by #2324