fix: Align the footer social links with the other Stackable sites

The set now matches the stackable.tech footer, which the hub footer also tracks:
LinkedIn, GitHub, YouTube and the news feed, in that order, plus Discord.
Discord appears on the hub but not on the homepage, so this is the first
Stackable footer to carry all five.

RSS comes from Font Awesome's solid set rather than brands, and each anchor
needs its own aria-label because the svg is aria-hidden and the link has no
text.

Author: lfrancke

ui/build.mjs

@@ -160,6 +160,9 @@ const icons = {
   search: 'solid/magnifying-glass',
   linkedin: 'brands/linkedin',
   github: 'brands/github',
+  youtube: 'brands/youtube',
+  rss: 'solid/rss',
+  discord: 'brands/discord',
   link: 'solid/link'
 };
 const symbols = Object.entries(icons).map(([name, faIcon]) => {

ui/src/partials/footer-content.hbs

@@ -63,6 +63,15 @@
           <a class="social-icon" href="https://github.com/stackabletech" target="_blank" aria-label="Stackable on GitHub">
             <svg class="svg-icon" aria-hidden="true"><use href="#icon-github"/></svg>
           </a>
+          <a class="social-icon" href="https://www.youtube.com/@stackabletech" target="_blank" aria-label="Stackable on YouTube">
+            <svg class="svg-icon" aria-hidden="true"><use href="#icon-youtube"/></svg>
+          </a>
+          <a class="social-icon" href="https://stackable.tech/en/feed/?category_name=news%2Cblog" target="_blank" aria-label="Stackable news and blog feed">
+            <svg class="svg-icon" aria-hidden="true"><use href="#icon-rss"/></svg>
+          </a>
+          <a class="social-icon" href="https://discord.com/invite/7kZ3BNnCAF" target="_blank" aria-label="Join the Stackable Discord community">
+            <svg class="svg-icon" aria-hidden="true"><use href="#icon-discord"/></svg>
+          </a>
         </div>
       </div>
       <p class="copyright-notice">