Add event calendar and update site content

Add a calendar page and ICS file for event subscriptions. Update the
Hello Linux presentation with Tux images and add setup instructions
to the README.

index.html

@@ -49,6 +49,13 @@
         <h1>FLUG Presentations</h1>
         <p>Welcome to the presentation repository for FLUG.</p>
 
+        <p>
+            <a
+                href="/calendar.html"
+                style="font-size: 1rem; font-weight: normal; color: #0366d6"
+                >📅 View Calendar & Add to yours</a
+            >
+        </p>
         <h2>Presentations</h2>
         <ul>
             <li>