Create Documentation for Template Creation

[ruflin]

See #639

• Introduce it in Getting Started Guide
  • Replace default by using beat config instead of script
  • Mention that if LS is used and not ES directly, still the script has to be used
• Add it to the configuration options

[ruflin]

@dedemorton I split this out from #639 as when I looked at the outputconfig it already looks quite packed and I wanted to discuss where to put these new options best.

[dedemorton]

@ruflin Soon I'll be restructuring the reference info about config options so that each section is a separate topic (to make each topic shorter and the information easier to retrieve). I'm also changing the TOC structure so that we can add some nested sub-topics.