content: Lighthouse doc for Total Blocking Time

[kaycebasques]

Author: @kaycebasques
Reviewer: TBD

Note that this is different than #1572. This is the doc that Lighthouse will link to and must be organized around that user journey. The doc that I'm creating here will eventually link to Phil's doc from #1572.

Target publish date: 2019-10-17 (Chrome 79 Branch Point)

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• 1. I've done my proposal
• 2. Proposal approved — reviewer checks this
• 3. I've added a draft to my proposal doc
• 4. Draft approved — reviewer checks this
• 5. I've submitted a pull request Add Total Blocking Time documentation for Lighthouse #1636
• 6. Pull request approved — reviewer checks this

[carolrivero]

Please confirm who the reviewer is. Thx.