Document new command line interface

[exekias]

New command line interface has been merged #4420

This adds subcommands for different actions while keeping backwards compatibility if no subcommand is given (it will execute run). We will need to document the new features and update existing invocations to the new format.

We may probably want to support documentation for beat specific subcommands in the future, apart from the default, common ones.

Right now we have:

$ metricbeat help
$ metricbeat version
$ metricbeat configtest
$ metricbeat setup
$ metricbeat run
$ metricbeat (alias for metricbeat run)

[tsg]

As part of #4506 I added a new flag --machine-learning to the setup command, which only loads the ML configurations. I defer the task of writing the docs for it to this ticket :).

[exekias]

#4510 was merged, this is the list of new commands:

$ metricbeat completion
$ metricbeat export config
$ metricbeat export template

@dedemorton let me know if you need any pointers when doing this task, we can do a call or I can write some notes

[exekias]

#4537 was merged, and it's the last one, it adds modules management command for metricbeat and filebeat:

$ metricbeat modules list
$ metricbeat modules enable apache2

[dedemorton]

Created #4832 to add these docs.

[dedemorton]

I think we can close this, right? Looks like maybe some of the names changed, but I think I covered all of this in the command ref info in 4832.