Replace defunct placekitten image hosts in demo

placekitten.com has shut down, so the demo images no longer load.
Point them at picsum.photos so images render in the demo and docs.

Closes #60

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mblode

docs/index.html

@@ -19,6 +19,21 @@
     .site-nav li { display: inline; padding: 0 16px; }
     .site-nav .brand { font-size: 1.5rem; }
     .hero { border-bottom: 1px solid rgba(0,0,0,0.12); padding-bottom: 16px; margin-bottom: 32px; }
+    .hero .button {
+      background-color: #3b82f6;
+      border: 1px solid transparent;
+      border-radius: 4px;
+      color: #fff;
+      display: inline-block;
+      font-weight: 400;
+      line-height: 1.5;
+      padding: 0.375rem 0.75rem;
+      text-align: center;
+      text-decoration: none;
+      transition: background-color 0.15s ease-in-out, border-color 0.15s ease-in-out;
+      white-space: nowrap;
+    }
+    .hero .button:hover { background-color: #2563eb; border-color: #2563eb; }
     footer { margin-bottom: 0; }
   </style>
   <script async src="https://www.googletagmanager.com/gtag/js?id=G-6D0Z3DW6BT"></script>
@@ -45,8 +60,8 @@ <h1>Marx</h1>
       <h3>The classless CSS reset (perfect for Communists).</h3>
       <p>No JavaScript. No Classes. Just raw CSS.</p>
       <div>
-        <a href="https://unpkg.com/marx-css/css/marx.min.css" target="_blank" rel="noopener"><button>Download</button></a>
-        <a href="https://github.com/mblode/marx" target="_blank" rel="noopener"><button>GitHub</button></a>
+        <a href="https://unpkg.com/marx-css/css/marx.min.css" class="button" target="_blank" rel="noopener">Download</a>
+        <a href="https://github.com/mblode/marx" class="button" target="_blank" rel="noopener">GitHub</a>
       </div>
     </center>
 
@@ -392,21 +407,21 @@ <h3>
             No <code>&lt;figure&gt;</code> element
           </h3>
           <p>
-            <img src="http://placekitten.com/480/480" alt="Image alt text">
+            <img src="https://picsum.photos/480/480" alt="Image alt text">
           </p>
           <h3>
             Wrapped in a <code>&lt;figure&gt;</code> element, no
             <code>&lt;figcaption&gt;</code>
           </h3>
           <figure>
-            <img src="http://placekitten.com/420/420" alt="Image alt text">
+            <img src="https://picsum.photos/420/420" alt="Image alt text">
           </figure>
           <h3>
             Wrapped in a <code>&lt;figure&gt;</code> element, with a
             <code>&lt;figcaption&gt;</code>
           </h3>
           <figure>
-            <img src="http://placekitten.com/420/420" alt="Image alt text">
+            <img src="https://picsum.photos/420/420" alt="Image alt text">
             <figcaption>Here is a caption for this image.</figcaption>
           </figure>
         </div>

index.html

@@ -337,21 +337,21 @@ <h3>
                 No <code>&lt;figure&gt;</code> element
               </h3>
               <p>
-                <img src="http://placekitten.com/480/480" alt="Image alt text">
+                <img src="https://picsum.photos/480/480" alt="Image alt text">
               </p>
               <h3>
                 Wrapped in a <code>&lt;figure&gt;</code> element, no
                 <code>&lt;figcaption&gt;</code>
               </h3>
               <figure>
-                <img src="http://placekitten.com/420/420" alt="Image alt text">
+                <img src="https://picsum.photos/420/420" alt="Image alt text">
               </figure>
               <h3>
                 Wrapped in a <code>&lt;figure&gt;</code> element, with a
                 <code>&lt;figcaption&gt;</code>
               </h3>
               <figure>
-                <img src="http://placekitten.com/420/420" alt="Image alt text">
+                <img src="https://picsum.photos/420/420" alt="Image alt text">
                 <figcaption>Here is a caption for this image.</figcaption>
               </figure>
             </div>