Docs: add Document Models / emmet tutorial (#917)

* Docs: add schema tutorial

* lnt

Author: rkingsbury

docs/_static/docs_store_Si_relax.json

@@ -4,6 +4,7 @@
 user/index
 user/install
 user/running-workflows
+user/docs-schemas-emmet
 user/fireworks
 user/atomate-1-vs-2
 user/codes/index