Left-hand navbar should highlight current page

[sarahmaddox]

For easy browsing, the left-hand navigation panel should highlight the page that's currently open, so readers can gain context from their position in the doc set.

[sarahmaddox]

/area docs

[sarahmaddox]

Note that the Docsy theme does highlight the current page title in the left-hand nav. See issue #291

[sarahmaddox]

/area docs-ux

[sarahmaddox]

The Docsy theme is now live on the site, and this issue is fixed.
/close

[k8s-ci-robot]

@sarahmaddox: Closing this issue.

In response to this:

The Docsy theme is now live on the site, and this issue is fixed.
/close

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