Add favicon to repo
As mentioned in #16 (closed), a favicon was missing, but it actually exists on cmucl.org. However, it doesn't exist for cmucl.common-lisp.dev. We need to check in the favicon so that it is available for both.
But while we're at it, we might want to consider updating the icon. Currently, it's a black lambda on a white background with some border lines. That's not how typical favicons look these days. I think this favicon looks better.