Add 'Fork Me On GitHub' ribbons

Since the Coala Community site is Open Source, I think it'd be a good idea to encourage contributions to it by adding a 'Fork Me On GitHub' ribbon to each page!

The ribbon, which could look something close to [these](https://github.com/blog/273-github-ribbons) would take the viewer to https://github.com/coala/community/fork where they'll be prompted to fork the repository and make changes :) I think it's important to have this as the site is about the community around Coala and GCI, both of which are founded around Open Source contributions :smile:

Examples of ribbons/links like this are often found on stuff like the [Docsify Site](https://docsify.js.org) and the [Jekyll Documentation](https://jekyllrb.com/docs/home/).