Add a basic tutorial for the tools in depot_tools.

docs/src/git-nav-downstream.txt

Index: docs/src/git-nav-downstream.txt
diff --git a/docs/src/git-nav-downstream.txt b/docs/src/git-nav-downstream.txt
index 9a41e0c693363515f50757ae4ae50df438492067..725017eb84bbbccd5951c488e6c478e46251951d 100644
--- a/docs/src/git-nav-downstream.txt
+++ b/docs/src/git-nav-downstream.txt
@@ -21,33 +21,7 @@ command will prompt you with a selection of the branches.
 
 EXAMPLE
 -------
-
-[subs="quotes,attributes"]
-----
-[white]**$ git map-branches**
-[red]**origin/master *{zwsp}**
-  [green]#cool_feature#
-    [green]#subfeature#
-  [green]#frozen_changes#
-  [green]#master#
-[fuchsia]#{NO UPSTREAM}#
-  [green]#bogus_noparent#
-[white]**$ git nav-downstream**
-
-Please select a downstream branch
-  0. cool_feature
-  1. frozen_changes
-  2. master
-Selection (0-2)[0]: 0
-[white]**$ git map-branches**
-[red]##origin/master##
-  [aqua]**cool_feature *{zwsp}**
-    [green]#subfeature#
-  [green]#frozen_changes#
-  [green]#master#
-[fuchsia]#{NO UPSTREAM}#
-  [green]#bogus_noparent#
-----
+demo:1[]
 
 include::_aliases.txt[]