fix(a11y): hide duplicated link text from screen readers

Because screen readers don't know that the anchor icon is not meant to be read,
they might read the title (which is the same as the heading itself) in addition
to the heading, thus causing a duplicated output.

Signed-off-by: Erik Michelson <github@erik.michelson.eu>

public/js/extra.js

@@ -845,6 +845,7 @@ function imgPlayiframe (element, src) {
 
 const anchorForId = id => {
   const anchor = document.createElement('a')
+  anchor.ariaHidden = 'true'
   anchor.className = 'anchor hidden-xs'
   anchor.href = `#${id}`
   anchor.innerHTML = '<i class="fa fa-link"></i>'