docs: Fix CONTRIBUTING link

This returns a 404 since the site is already generated from the docs
directory. Furthermore, the `CONTRIBUTING.md` markdown file isn't in the
generated site, just the HTML.

Instead, use jekyll's `link` tag to create the link. Unfortunately,
before jekyll 4.0 (github-pages uses 3.9), you have to prepend the base
URL.

docs/index.md

@@ -143,7 +143,7 @@ make install DESTDIR=/path/to/dest
 
 ## Contributing
 
-See [Contributing](docs/CONTRIBUTING.md).
+See [Contributing]({{ site.baseurl }}{% link CONTRIBUTING.md %}).
 
 ## Licensing