Create new tutorial for A/B testing

Questions about A/B testing come up all the time, and people are apparently don't have a solid understanding of how A/B testing works.

The issue names no file, test, or entry point; begin by reviewing the site's existing tutorial structure and clarify the intended audience and scope for A/B testing. Done means a published tutorial that gives readers a solid understanding of how A/B testing works.

Written by the indexing model from the issue text.