docs: Add Contributing Tutorial to Mkdocs pages

This adds the Contributing Tutorial (contributing-tutorial.md) to
the pages setting of Mkdocs, so that the tutorial will render
in the readthedocs.io documentation.

Closes: #1711
Approved by: jlebon

docs/CONTRIBUTING.md

@@ -168,7 +168,7 @@ Instead do this:
           }
       }
 
-Contributing: Tutorial
-----------------------
+Contributing Tutorial
+---------------------
 
-For a detailed walk-through on building, modifying, and testing, see this [tutorial on how to start contributing to OSTree](contributing-tutorial.md).
+For a detailed walk-through on building, modifying, and testing, see this [tutorial on how to start contributing to OSTree](contributing-tutorial.md).