Add accessible name to navigation icon links (#442)

Currently these links lack an accessible name at large screen sizes (as their actual text content is set in the `<small class="d-lg-none ...">`).
Adding `aria-label` to the links themselves explicitly circumvents this problem.

Additionally, this fixes the visible text for the "Twitter" link.

src/layouts/partials/navbar.html

@@ -40,19 +40,19 @@ <h5 class="offcanvas-title text-white" id="bdNavbarOffcanvasLabel">Bootstrap</h5
 
         <ul class="navbar-nav flex-row flex-wrap ms-md-auto">
           <li class="nav-item col-6 col-lg-auto">
-            <a class="nav-link py-2 px-0 px-lg-2" href="{{ .Site.Params.github_org }}" target="_blank" rel="noopener">
+            <a class="nav-link py-2 px-0 px-lg-2" href="{{ .Site.Params.github_org }}" target="_blank" rel="noopener" aria-label="GitHub">
               {{ partial "icons/github.svg" (dict "class" "navbar-nav-svg" "width" "16" "height" "16") }}
               <small class="d-lg-none ms-2">GitHub</small>
             </a>
           </li>
           <li class="nav-item col-6 col-lg-auto">
-            <a class="nav-link py-2 px-0 px-lg-2" href="https://twitter.com/{{ .Site.Params.twitter }}" target="_blank" rel="noopener">
+            <a class="nav-link py-2 px-0 px-lg-2" href="https://twitter.com/{{ .Site.Params.twitter }}" target="_blank" rel="noopener" aria-label="X (formerly Twitter)">
               {{ partial "icons/twitter.svg" (dict "class" "navbar-nav-svg" "width" "16" "height" "16") }}
-              <small class="d-lg-none ms-2">Twitter</small>
+              <small class="d-lg-none ms-2">X (formerly Twitter)</small>
             </a>
           </li>
           <li class="nav-item col-6 col-lg-auto">
-            <a class="nav-link py-2 px-0 px-lg-2" href="{{ .Site.Params.opencollective }}" target="_blank" rel="noopener">
+            <a class="nav-link py-2 px-0 px-lg-2" href="{{ .Site.Params.opencollective }}" target="_blank" rel="noopener" aria-label="Open Collective">
               {{ partial "icons/opencollective.svg" (dict "class" "navbar-nav-svg" "width" "16" "height" "16") }}
               <small class="d-lg-none ms-2">Open Collective</small>
             </a>