docs(website): add Essential Primer PDF to download page

Author: KrisSimon

Website/src/download.html

@@ -412,6 +412,17 @@ <h3>Construction Studies</h3>
                     <span class="version">ARO-Construction-Studies.pdf</span>
                 </div>
 
+                <div class="download-card">
+                    <div class="platform-icon">&#x1F4D3;</div>
+                    <h3>Essential Primer</h3>
+                    <p class="platform-desc">A concise introduction to ARO for developers who want to get up to speed quickly</p>
+                    <a href="https://github.com/arolang/aro/releases/latest/download/ARO-Essential-Primer.pdf" class="download-btn">
+                        <svg viewBox="0 0 16 16" fill="currentColor"><path d="M2.75 14A1.75 1.75 0 011 12.25v-2.5a.75.75 0 011.5 0v2.5c0 .138.112.25.25.25h10.5a.25.25 0 00.25-.25v-2.5a.75.75 0 011.5 0v2.5A1.75 1.75 0 0113.25 14H2.75z"/><path d="M7.25 7.689V2a.75.75 0 011.5 0v5.689l1.97-1.969a.749.749 0 111.06 1.06l-3.25 3.25a.749.749 0 01-1.06 0L4.22 6.78a.749.749 0 111.06-1.06l1.97 1.969z"/></svg>
+                        Download PDF
+                    </a>
+                    <span class="version">ARO-Essential-Primer.pdf</span>
+                </div>
+
                 <div class="download-card">
                     <div class="platform-icon">&#x1F9E9;</div>
                     <h3>The Plugin Guide</h3>