When hovering a heading, make the #-anchor fully opaque (#1949)

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.

src/pydata_sphinx_theme/assets/styles/base/_base.scss

@@ -39,17 +39,11 @@ a {
   // The # anchor that appears on hover over headings
   &.headerlink {
     color: var(--pst-color-secondary);
-    opacity: 0.7;
     font-size: 0.8em;
     padding: 0 4px;
     margin-left: 0.2em;
     text-decoration: none;
-    transition: all 0.2s ease-out;
     user-select: none;
-
-    &:hover {
-      opacity: 1;
-    }
   }
 
   // set up a icon next to the shorten links from github and gitlab