Add `--help `docs for meltano CLI

Several of the CLI commands are not documented in meltano --help. Adding descriptions here could help streamline the new user experience.

Also, many of the descriptions are trailing off in the screenshot above '...'.

CLI documentation for reference: https://meltano.com/docs/command-line-interface.html

Reported in slack: https://meltano.slack.com/archives/C01TCRBBJD7/p1618932986365900?thread_ts=1618925301.362900&cid=C01TCRBBJD7

Update 2021-11-30

This is much better now but still has some missing hints.

$ meltano --help
Usage: meltano [OPTIONS] COMMAND [ARGS]...

  Get help at https://www.meltano.com/docs/command-line-interface.html

Options:
  --log-level [debug|info|warning|error|critical]
  -v, --verbose
  --environment TEXT              Meltano environment name
  --version                       Show the version and exit.
  --help                          Show this message and exit.

Commands:
  add          Add a plugin to your project.
  config       Display Meltano or plugin configuration.
  discover
  elt          meltano elt EXTRACTOR_NAME LOADER_NAME extractor_name: Which...
  environment  Manage Environments.
  init         Creates a new Meltano project
  install      Installs all the dependencies of your project based on the...
  invoke       Invoke the plugin's executable with specified arguments.
  remove       Remove a plugin from your project.
  schedule     Manage Schedules.
  schema
  select       Execute the meltano select command.
  ui
  upgrade
  user