Documentation: when focused on header anchor (#) links they are not shown

[rouilj]

Your headers have # links that show up when hovering over the header.
However if somebody is tabbing through the document, these links are
focused but they are not visible. This leaves the user wondering where the focus
is.

Changing the css to read:

:is(h2, h3):hover .heading-anchor, .heading-anchor:focus {
   opacity: 1
}

seems to work for me.

-- rouilj