Use favicon to avoid error message

[conorsch]

The site's lack of a favicon generates a 404 error in the JS console:

Since once of the tutorial's first steps is to open the console, the red error message is shaping first-time students' experience. In fact, I'm reporting this issue on behalf of a first-time student who encountered the error and assumed the rest of the tutorial would not work.

In #55 it was noted that the site uses GitHub Pages for hosting. It looks like custom favicons are possible even with GitHub Pages:

 <link rel="shortcut icon" type="image/x-icon" href="favicon.ico">