[docs] Explicitly link the Idris2 tutorial

This is likely the closest one can come to a "Type-Driven Development
with Idris **2**" book, so it should be near the top.

Huge props to Stefan Höck for writing the original, and to the community
for turning it into an mdBook and now having stewardship of it.

Author: CodingCellist

docs/source/tutorial/conclusions.rst

@@ -10,6 +10,9 @@ dependent types in general, can be obtained from various sources:
 *  `Type-Driven Development with Idris <https://www.manning.com/books/type-driven-development-with-idris>`_
    by Edwin Brady, available from `Manning <https://www.manning.com>`_.
 
+*  `The community Idris2 tutorial <https://idris-community.github.io/idris2-tutorial/>`_,
+   originally written by Stefan Höck, now maintained by the community.
+
 *  The Idris website (https://www.idris-lang.org/) has pointers to a broad range
    of resources.