Add app description in CLI Reference Docs

[D3nn]

Currently, the CLI Reference docs has no description for app when running the command wego. This needs to be added to give context for the app command. The full output should be:

Weave GitOps

Usage:
  wego [command]

Available Commands:
  app         Use app commands
  flux        Use flux commands
  help        Help about any command
  install     Install or upgrade Wego
  version     Display wego version

Flags:
  -h, --help               help for wego
      --namespace string   gitops runtime namespace (default "wego-system")
  -v, --verbose            Enable verbose output

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