Modify the build system for contrib documentation

[danielfernandez]

In order to integrate community-contributed documentation like #12, we should be able to create documents using a different set of metadata items.

This is because, for example, contributed translations of the tutorials will not always refer to the latest version of thymeleaf (${project.version}, as it is now), and their latest update date will of course not match that of the original English official documents. So these contributed docs should be able to specify at least their own project version and date so that HTML templates can use them.

In order to add these metadata items, a new format (markdown extension) must be adopted. See #16

[danielfernandez]

Closed by @1ad544bf9799cef0daba8fba616078a94f48269d. Actions taken:

Modified the Gradle build script (build.gradle) and also the folder structure so that:

• Documents will now live in doc/[articles|tutorials] instead of directly inside doc.
• Tutorials will now be classified per Thymeleaf version: 2.1, 2.1.4 or 2.1.x, so that we can have two or more versions of the tutorials online at the same time. New path: /doc/tutorials/2.1/whatever.html
• Redirections have been added for the old locations of doc files, pointing to the new ones. Old redirections have also been updated.
• Contrib tutorial translations will be in the same folder as official versions, but will have a different set of metadata.

[danielfernandez]

This should finally allow publishing #12