Add an ARIA category to the rules index page

[WilcoFiers]

We've just merged a couple of ARIA approved rules. Those are currently listed under WCAG. That is reasonable, as those have WCAG criteria as secondary requirements, but it would be better to give ARIA its own section in the rules index page.

[daniel-montalvo]

Thanks much.

@WilcoFiers Addressed this in #262 . For future we should probably update the act_rules layout and the wcag-mapping.json so that this new ARIA list is fully automated.