Tutorial: Provide a source archive when published on cmake.org

Provide readers following the tutorial on `cmake.org` a direct link to
download the tutorial's source examples.  Previously readers had to
fetch the CMake source tree separately and look for the tutorial source
examples inside it.

Fixes: #22475

Help/guide/tutorial/index.rst

@@ -11,8 +11,9 @@ work together in an example project can be very helpful.
 Steps
 =====
 
-The tutorial documentation and source code for examples can be found in
-the ``Help/guide/tutorial`` directory of the CMake source code tree.
+.. include:: source.txt
+
+|tutorial_source|
 Each step has its own subdirectory containing code that may be used as a
 starting point. The tutorial examples are progressive so that each step
 provides the complete solution for the previous step.

Help/guide/tutorial/source.txt

@@ -0,0 +1,3 @@
+.. |tutorial_source| replace::
+  The tutorial documentation and source code examples can be found in
+  the ``Help/guide/tutorial`` directory of the CMake source code tree.