[A11Y] add `aria-current` to navigation lists

[aghassemi]

On the main ampstart.com, where-ever the CSS www-current-page exists, we also need to add aria-current="page"

for templates, similar whenever a navlink is "highlighted", aria-current="page" should also be added.

[aghassemi]

(will need to wait for https://github.com/ampproject/amphtml/pull/12102 to be in prod. ETA ~3 weeks)

[camelburrito]

i just got rid ofwww-current-page in the recent updates. But there could be a way to add the aria. Thanks for filing