When hovering a heading, make the #-anchor fully opaque

[gabalafou]

Previously, when the heading was hovered but not the actual hash-anchor itself, the anchor would be 70% opaque. Hovering the anchor itself would bring it to 100% opacity.

However, at 70% opacity, the contrast between the #-anchor and the background did not meet accessibility contrast requirements (for normal text, WCAG AA requires 4.5:1 contrast ratio).

This PR makes it so that it's always 100% opaque, whether you hover the heading or the actual anchor itself.

This means that the anchor itself does not have a perceivable hover state, but I'm not sure it needs one. If we decide that it does need a hover state, we can add one later. At any rate, the previous 30% change in opacity to mark the hover state (hovering the heading to hovering the #-anchor) was too subtle to meet accessibility requirements.

Fixes external issue Quansight-Labs/czi-scientific-python-mgmt#96.

[github-actions]

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