doc/admin: add reference to other intro material

Problem: the admin guide was written to be standalone, but other
flux-core docs may be helpful.

Add a pointer to the intro stuff.

Author: garlick

doc/guide/admin.rst

@@ -20,6 +20,11 @@ resource manager on a cluster.
 Overview and Background
 ***********************
 
+:doc:`start` and :doc:`interact` provide recipes for starting Flux and
+navigating a hierarchy of Flux instance that do not require administrator
+privilege or configuration.  It may be helpful to develop some perspective
+on Flux in these contexts before configuring a Flux system instance.
+
 Flux Architecture
 =================