fix: doc cards hover state apply even when card is not a link

[wesleyboar]

Overview

Doc cards have hove hover states that should only occur for cards.

Changes

• required by enhance: some accountstatus cards as links TACC-Docs#68

Testing

1. Open a TACC doc site that has cards that are not links.
   ^{e.g. via enhance: some accountstatus cards as links TACC-Docs#68}
2. Live edit content of dist/core-styles.docs.css to use the one from this PR.
3. Hover over card, not heading.
4. Verify no change e.g. no underline for heading text.
5. Hover over heading.
6. Verify heading permalink behavior.

UI

Before.mov

After.mov