fix tutorial markdown-rendering issue and broken buttons

The markdown wasn't being correctly processed for `index` or `the_end`. Also, the button .tutorial-page divs added in 5bc2836 were overlaying the 'improve this doc' buttons, preventing them from being clickable. Lastly, the 'improve this doc' button URLs were wrong, 404ing at github. (Most of this request essentially rolls back 5bc2836)