fix: add a description to the root command

[obs-gh-mattcotter]

Description

OB-37820 Here's how it looks now:

$ ./observe-agent help
Observe distribution of OTEL Collector along with CLI utils to help with setup
and maintenance. To start the agent, run: observe-agent start

Usage:
  observe-agent [command]

Available Commands:
  completion  Generate the autocompletion script for the specified shell
  diagnose    Run diagnostic checks.
  help        Help about any command
  init-config Initialize agent configuration
  start       Start the Observe agent process.
  status      Display status of agent
  version     Display the currently installed version of the observe-agent.

Flags:
      --config string   config file path
  -h, --help            help for observe-agent
  -t, --toggle          Help message for toggle

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