docs(nav): Highlight current nav-index-listing

[kentcdodds]

Color the current nav-index-listing item dark red to make it easier to know where you are.

Closes #9970

[mary-poppins]

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[pkozlowski-opensource]

@kentcdodds thnx for the PR. At present it seems to be breaking some e2e tests - at the beginning I thought it was Travis flake but after restarting the job a couple of times it looks like a legitimate bug. Could you please have a look? https://travis-ci.org/angular/angular.js/jobs/40415397

[kentcdodds]

Looking into it now. This is why we love Travis 👍

[kentcdodds]

Looks like that fixed it @pkozlowski-opensource :-)

[pkozlowski-opensource]

LGTM, merging

[pkozlowski-opensource]

Hmm, I'm just wondering what the active class is used for? It seems to be added to all the links but doesn't seem to be defined anywhere in the CSS. On top of this the condition for the "active" class looks similar to the "current" check.

I'm going to merge this one as the proposed highlighting is definitively a good thing, but it would be cool to clean-up stuff if we can.