Restlet.org links including a ".md" suffix are broken

[jlouvel]

Thierry, we should convert those links to remove the ".md" suffix (needed for proper GitHub navigation of docs) when publishing the HTML pages for Restlet.org

See example of broken links here:
http://restlet.org/learn/guide/2.2/introduction/first-steps/

[jlouvel]

Think also about the index pages, currently as "index.md"