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.

20260403-HelloLinux/index.html

@@ -23,6 +23,34 @@
                     <p>April 3, 2026</p>
                 </section>
 
+                <section>
+                    <h2>Meet Tux, the Linux Mascot!</h2>
+                    <div
+                        style="
+                            display: flex;
+                            justify-content: center;
+                            align-items: center;
+                            gap: 20px;
+                        "
+                    >
+                        <img
+                            src="tux1.png"
+                            alt="Tux 1"
+                            style="max-height: 300px; object-fit: contain"
+                        />
+                        <img
+                            src="tux2.png"
+                            alt="Tux 2"
+                            style="max-height: 300px; object-fit: contain"
+                        />
+                        <img
+                            src="tux3.png"
+                            alt="Tux 3"
+                            style="max-height: 300px; object-fit: contain"
+                        />
+                    </div>
+                </section>
+
                 <section>
                     <h2>What is a computer?</h2>
                     <img