fix(dev-landing): use relative paths for book and tinytorch links

Absolute paths like /book/ resolve incorrectly on GitHub Pages subpath
deployments (cs249r_book_dev). Using relative paths (./book/) ensures
the links work regardless of the hosting location.

.github/dev-landing/index.html

@@ -132,14 +132,14 @@
         <p class="subtitle">Machine Learning Systems • Development Environment</p>
         
         <div class="cards">
-            <a href="/book/" class="card">
+            <a href="./book/" class="card">
                 <div class="card-icon">📖</div>
                 <h2>Textbook</h2>
                 <p>Machine Learning Systems: Principles and Practices of Engineering Artificially Intelligent Systems</p>
                 <span class="card-link">Open Book Preview →</span>
             </a>
             
-            <a href="/tinytorch/" class="card">
+            <a href="./tinytorch/" class="card">
                 <div class="card-icon">🔥</div>
                 <h2>Tiny🔥Torch</h2>
                 <p>Build ML Systems from Scratch: An interactive course for implementing your own PyTorch-style framework</p>