TOC Menu sidbar (should use aria-current)

The currently active link on the Table of Contents Menu on left sidebar should have aria-current="true". it currently has the class .is-active, so just need to add it ti that same a tag.

e.g. https://antora.gitlab.io/antora-ui-default/#liber-recusabo

should have:

<a href="#liber-recusabo" class="is-active" aria-current="true">
  Liber recusabo
</a>

REF:

https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Reference/Attributes/aria-current#true

#a11y