tutorial: move tutorial to standalone site (#13450)

* docs: add a spack environment for building the docs

* docs: remove tutorial and link to spack-tutorial.readthedocs.io

The tutorial now has its own standalone website, versioned by instances
of the tutorial.  Link to that instead of versioning it directly with Spack.

lib/spack/docs/conf.py

@@ -159,7 +159,7 @@ def setup(sphinx):
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
-exclude_patterns = ['_build', '_spack_root']
+exclude_patterns = ['_build', '_spack_root', '.spack-env']
 
 # The reST default role (used for this markup: `text`) to use for all documents.
 #default_role = None