docs: remove placeholder application-lifecycle page and complete first draft of embedding docs

- Delete the empty "Application Lifecycle" page (was just a TODO placeholder)
- Expand "Storing C Data" from stub to full documentation covering foreign classes, initialization, access, and finalization
- Add note that VM is not re-entrant; warn against calling wrenCall/wrenInterpret from foreign methods
- Fix broken cross-reference links (configuring-the-vm.html, wren.hpp include)
- Clarify slot API usage: remove confusing sentence about implicit return of receiver
- Fix grammar: "each VM them differently" → "each VM differently", "is only be called" → "is only called"
- Improve phrasing throughout for clarity and consistency

doc/site/embedding/template.html

@@ -29,7 +29,6 @@
         <li><a href="calling-c-from-wren.html">Calling C from Wren</a></li>
         <li><a href="storing-c-data.html">Storing C Data</a></li>
         <li><a href="configuring-the-vm.html">Configuring the VM</a></li>
-        <li><a href="application-lifecycle.html">Application Lifecycle</a></li>
       </ul>
     </section>
   </nav>
@@ -49,7 +48,6 @@
             <li><a href="calling-c-from-wren.html">Calling C from Wren</a></li>
             <li><a href="storing-c-data.html">Storing C Data</a></li>
             <li><a href="configuring-the-vm.html">Configuring the VM</a></li>
-            <li><a href="application-lifecycle.html">Application Lifecycle</a></li>
           </ul>
         </td>
         <td>