docs: split monolithic core-library.markdown into per-class docs under doc/site/core/

doc/site/template.html

@@ -13,7 +13,7 @@
 <header>
   <div class="page">
     <div class="main-column">
-      <h1><a href="index.html">wren</a></h1>
+      <h1><a href="./">wren</a></h1>
       <h2>a classy little scripting language</h2>
     </div>
   </div>
@@ -47,7 +47,7 @@
     <section>
       <h2>reference</h2>
       <ul>
-        <li><a href="core-library.html">Core Library</a></li>
+        <li><a href="core">Core Library</a></li>
         <li><a href="embedding-api.html">Embedding API</a></li>
         <li><a href="performance.html">Performance</a></li>
         <li><a href="contributing.html">Contributing</a></li>