Turn the listing into one table

The page looked messy when each section was a separate table with
different column widths than the others.

The W3C HTML Validator now gives the following error, but I cannot
figure out how to get around it without complicated CSS.

The element “h2” must not appear as a descendant of the “th” element.

www/index.html

@@ -31,8 +31,12 @@
   </header>
   <h1 id="logo">Scheme Conservatory</h1>
   <p>Preservation and maintenance of old software and websites.</p>
-  <h2>Websites</h2>
   <table class="no-border">
+    <tr>
+      <th colspan="2">
+        <h2>Websites</h2>
+      </th>
+    </tr>
     <tr>
       <th>
         <a href="schemers/">Schemers.org</a>
@@ -58,9 +62,11 @@
       </th>
       <td>Hosted at Indiana University</td>
     </tr>
-  </table>
-  <h2>Implementations</h2>
-  <table class="no-border">
+    <tr>
+      <th colspan="2">
+        <h2>Implementations</h2>
+      </th>
+    </tr>
     <tr>
       <th>
         <a href="dream/">Dream</a>
@@ -127,9 +133,11 @@
       </th>
       <td>R4RS for the VxWorks RTOS</td>
     </tr>
-  </table>
-  <h2>Libraries</h2>
-  <table class="no-border">
+    <tr>
+      <th colspan="2">
+        <h2>Libraries</h2>
+      </th>
+    </tr>
     <tr>
       <th>
         <a href="ffigen/">FFIGEN</a>
@@ -148,9 +156,11 @@
       </th>
       <td>Portable syntax-case with implicit phasing</td>
     </tr>
-  </table>
-  <h2>Applications</h2>
-  <table class="no-border">
+    <tr>
+      <th colspan="2">
+        <h2>Applications</h2>
+      </th>
+    </tr>
     <tr>
       <th>
         <a href="dreamos/">DreamOS</a>
@@ -171,9 +181,7 @@
     </tr>
   </table>
   <h2>Contributing</h2>
-  <th>
-    <a href="https://gitea.scheme.org/conservatory">Repositories at
-    <code>gitea.scheme.org</code></a>
-  </th>
+  <p><a href="https://gitea.scheme.org/conservatory">Repositories
+  at <code>gitea.scheme.org</code></a></p>
 </body>
 </html>