Added API reference

Author: MissCorruption

docs/_layouts/default.html

@@ -22,6 +22,7 @@ <h2>SexLab P+</h2>
           <li><a href="{{ '/slp/installation/' | relative_url }}">Installation</a></li>
           <li><a href="{{ '/slp/how-to-play/' | relative_url }}">How to Play</a></li>
           <li><a href="{{ '/slp/settings-reference/' | relative_url }}">Settings Reference</a></li>
+          <li><a href="{{ '/slp/api-reference/' | relative_url }}">API Reference</a></li>
         </ul>
 
         <h2>Migration Guide</h2>