Admin guide landing page added

Added the admin guide in the mistral document index file as per
the doc-migration spec.

Change-Id: I821257f5be8b6778b1a6eaf9c138a06cef8b2792
Closes-Bug: #1706449

doc/source/admin/index.rst

@@ -0,0 +1,7 @@
+Mistral Admin Guide
+=====================
+
+.. toctree::
+   :maxdepth: 1
+
+   upgrade_guide

doc/source/index.rst

@@ -28,7 +28,6 @@ User guide
 
    install/index
    configuration/index
-   admin/upgrade_guide
 
 **API**
 
@@ -59,6 +58,14 @@ Developer guide
 
    contributor/index
 
+Admin guide
+-----------
+
+.. toctree::
+   :maxdepth: 2
+
+   admin/index
+
 Indices and tables
 ==================