docs: add Getting Started page with setup instructions and interactive mode examples

Add a comprehensive Getting Started guide covering Unix/Mac installation via git clone and make, interactive mode usage with code examples, and script execution with a Mandelbrot fractal demo. Also update the site navigation template to link the new page, add placeholder pages for Embedding, Error Handling, and Getting Involved, fix the index page link from "usage.html" to "embedding.html", and reorder the types section in the nav to list Fibers before Functions.

doc/site/template.html

@@ -18,7 +18,7 @@
   <div class="nav">
     <h2>welcome</h2>
     <ul>
-      <li>Getting Started</li>
+      <li><a href="getting-started.html">Getting Started</a></li>
     </ul>
     <h2>language</h2>
     <ul>
@@ -27,27 +27,24 @@
       <li><a href="variables.html">Variables</a></li>
       <li><a href="branching.html">Branching</a></li>
       <li><a href="looping.html">Looping</a></li>
+      <li><a href="error-handling.html">Error Handling</a></li>
     </ul>
     <h2>types</h2>
     <ul>
       <li><a href="values.html">Values</a></li>
       <li><a href="classes.html">Classes</a></li>
-      <li><a href="functions.html">Functions</a></li>
       <li><a href="fibers.html">Fibers</a></li>
+      <li><a href="functions.html">Functions</a></li>
       <li><a href="lists.html">Lists</a></li>
       <li><a href="maps.html">Maps</a></li>
     </ul>
-    <h2>usage</h2>
-    <ul>
-      <li>Concurrency</li>
-      <li>Error Handling</li>
-      <li>Standalone</li>
-      <li>Embedding</li>
     </ul>
     <h2>other</h2>
     <ul>
       <li><a href="qa.html">Q &amp; A</a></li>
       <li><a href="performance.html">Performance</a></li>
+      <li><a href="embedding.html">Embedding</a></li>
+      <li><a href="getting-involved.html">Getting Involved</a></li>
     </ul>
   </div>
   <div class="content">