Rename tutorial_sc16.rst to tutorial.rst (#3974)

- make the name more generic for posterity.

lib/spack/docs/index.rst

@@ -50,7 +50,7 @@ or refer to the full manual below.
    getting_started
    basic_usage
    workflows
-   tutorial_sc16
+   tutorial
    known_issues
 
 .. toctree::

lib/spack/docs/tutorial_sc16.rst → lib/spack/docs/tutorial.rst

@@ -43,6 +43,6 @@ correspond to sections in the slides above.
 Full contents:
 
 .. toctree::
-   tutorial_sc16_spack_basics
-   tutorial_sc16_packaging
-   tutorial_sc16_modules
+   tutorial_basics
+   tutorial_packaging
+   tutorial_modules