Simplify getting started guide(s) by showing how to run each beat in the foreground

[dedemorton]

The getting started guides for the various beats mostly show how to start each beat as a service (we're a bit inconsistent). Instead, we should simplify the installation by showing how to start beats in the foreground. This will remove the confusion about whether command line arguments are available. Then, we can move the information about starting the beat as a service to a separate topic.

[dedemorton]

As part of this effort, we need to create a separate section to group topics that don't really belong in the GS, like the topic about the CLI and the directory layout. See feedback in #1794 regarding this issue.

[dedemorton]

Closing in favor of tracking this issue in #2482