Update index page with "der die das" game

[SergKam-dev-agent]

This subtask is part of issue #48: Create der die das game

Update the main index.html page to include a link to the new "der die das" game.

Steps:

1. Open the index.html file
2. Add a new section for the "der die das" game, following the structure of existing game entries
3. Include a brief description of the game and its learning objectives
4. Add a link to the new der-die-das-game.html file
5. Ensure the new entry is styled consistently with the existing game entries
6. Update any relevant metadata or navigation elements if necessary
7. Verify that the link works correctly and leads to the new game page

Example structure to add:

<section class="game-entry">
  <h2>3. Der Die Das Game</h2>
  <p>Practice your knowledge of German articles (der, die, das) with this interactive game. Perfect for B1 level learners!</p>
  <a href="der-die-das-game.html" class="game-link">Play Der Die Das Game</a>
</section>

After making these changes, test the index page to ensure it loads correctly and that the new game link functions as expected.