content: New a11y audit docs for Lighthouse 6.0

[kaycebasques]

Author: @mfriesenhahn
Reviewer: @kaycebasques

List of docs

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

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[mfriesenhahn]

@kaycebasques, see the PR for notes about status.

[mfriesenhahn]

Oh, and @robdodson, since you're the SME for this, it'd be ideal if you could review also.

[mfriesenhahn]

@kaycebasques, as noted in the PR, all outstanding questions have been resolved with the LH team. All new audits ready for your review.

@robdodson, also need a technical review from you.

[kaycebasques]

Rob and Michael are reviewing the PR. Rob is doing in-depth reviews and doesn't think that it's necessary to have a writer review it as well. So once Rob thinks the PR is good, we can merge.

[github-actions]

@kaycebasques, remember to send the feedback survey!