you can use  include <?php include 'tags.html' ?> but i don't understand you need  if you write in tags.html  <?php echo $page['tags']; ?> you get tags but you need  write tags first in  post.md like this:

 Title: Welcome  
 Tags: tag1, tag2

Try to use this code

<p>Tags: <?php if($page['tags']){
                    echo $page['tags'];
                }else{
                    echo 'No tags in this post';
                }; ?></p>

stradfred, there is no tags cloud functionality, yet. It can be implemented as a plugin or built function.