create internal training videos for dhall based sourcegraph.com pipeline

Created by: ggilmore

Part of https://github.com/sourcegraph/sourcegraph/issues/13340

Create training video + documentation for Sourcegraph teammates to learn how to interact with the new sourcegraph.com dhall-based pipeline. It's similar in spirit to https://github.com/sourcegraph/sourcegraph/issues/14646, but it'll include more dev environment + directed setup since it's a more in depth tutorial than the overview videos for customers.