Remaining work for Beats Dev Guide:

• Decide how far to backport this change. Decision: Don't backport. This is for 6.0.0+.
• Set up redirects for moved content
• Replace CONTRIBUTING.md content with a link to the section in the dev guide
• Update any links that point to CONTRIBUTING.md to point to the docs instead (for example https://github.com/elastic/beats/README.md should point to the published docs).
• Update section about adding packetbeat protocols so that it has actual content
• Add conceptual info that covers various extension points for beats (work in content from the Elastic{ON} presentation (see Restructure Beats documentation #3737 for an idea of the propsed TOC)
• Consider whether we should take the content from this blog and add it to the devguide: https://www.elastic.co/blog/build-your-own-beat