New rule: First focusable element is link to non-repeated content

[Jym77]

Extracting the separate rule from #421 .

Closes issue(s):

• N/A

Need for Final Call: This will require a 2 weeks Final Call (new rule)

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[Jym77]

Summoning @carlosapaduarte and @WilcoFiers to continue the discussion on actionable element.

[Jym77]

@WilcoFiers good catch on the presence of repeated content. That was lost in various iterations… This lead me to make a composite rule with one atomic checking for repeated content and another for skip link.