New documentation for Launchable concepts

We're launching a new section of our product documentation called Concepts. This section explains the various concepts in Launchable's object model, including Workspaces, Builds, Test Sessions, and Subsets, in detail.

We hope the new pages are useful to anyone looking for more info about how the various pieces fit together!

Check them out here: https://docs.launchableinc.com/concepts/