link remaining a11y audits to aXe docs

[kaycebasques]

The current plan is to link all of the a11y audits to their respective sources in the aXe docs.

These audits currently link to references I wrote before the a11y category ballooned to 50+ audits.

Deprecating the a11y docs I wrote reduces my workload while also ensuring a consistent UX (if you click on "Learn more" in an a11y doc now, you can always expect to land on an aXe doc)

[brendankenny]

@robdodson said he was good with this change.

The few docs pages I checked don't appear to be different, but should we be linking to https://dequeuniversity.com/rules/axe/2.2/* instead of 1.1 since 2.2.1 is the version of axe-core we're now using?

[paulirish]

@kaycebasques i gave you a PR for that small change: https://github.com/kaycebasques/lighthouse/pull/1

looks like i can't update your fork directly so.. :) wanna merge my PR?

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[kaycebasques]

@paulirish done