doc: add glossary references

Problem: glossary terms are not referenced when used in
guide documents.

Reference them on first use in each document.

doc/guide/interact.rst

@@ -166,9 +166,9 @@ socket and looks something like::
 
   local:///tmp/flux-lMDa6Z/local-0
 
-In the *initial program* (batch script, interactive alloc shell, or whatever),
-the FLUX_URI environment variable is set to the local URI of the rank 0
-broker.  Flux commands in the initial program, which also runs on rank 0,
+In the :term:`initial program` (batch script, interactive alloc shell, or
+whatever), the FLUX_URI environment variable is set to the local URI of the rank
+0 broker.  Flux commands in the initial program, which also runs on rank 0,
 read FLUX_URI and reference the instance that started them.
 
 When running outside of an instance, FLUX_URI will not be set.  In this case,

doc/guide/start.rst

@@ -22,7 +22,7 @@ parallel, as a *parallel program* if you will.
 phases:
 
 #. Initialize
-#. The rank 0 broker runs the **initial program** to completion
+#. The rank 0 broker runs the :term:`initial program` to completion
 #. Finalize
 
 By default, the initial program is an interactive shell.  If :man1:`flux-start`