docs: Fix CONTRIBUTING link

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.