BYS: Move Lifecycle of a Launch into Handbook

[dylanirlbeck]

Part of 18F/before-you-ship#421

Per Aidan's suggestion, I'm going to break up moving the BYS content into the Handbook into multiple PRs.

This one focuses on the Lifecycle of a Launch page. A short GIF is included below demonstrating the new page running locally.

Questions:

• Is there a better location for the BYS content on the main Handbook page?
• Did I consolidate the various tabs on the Lifecycle BYS page properly? I collapsed the content into one Markdown file, but if there is a better structure I'm open to suggestions.

[adborden]

Looks like a great start!

Some assumptions/discussion for the future (from our group review):

• we probably want to drop the name "before you ship" "Before You Ship" implies that the guide is only needed last-minute before-you-ship#305
• we probably want to integrate the content more into the rest of the guide, instead of b-y-s section as it exists today
• audience is more than just engineering