Reorganize docs into user/admin guide

Refresh the user and admin guide for v3 changes, and reorganize into
a narrative structure which makes more sense for v3.

Change-Id: I4ac3b18d5ed33b0fea4e2ef0318b19bfc3447ccc
diff --git a/doc/source/user/index.rst b/doc/source/user/index.rst
new file mode 100644
index 0000000..3eca04b
--- /dev/null
+++ b/doc/source/user/index.rst
@@ -0,0 +1,18 @@
+User's Guide
+============
+
+This guide is for all users of Zuul.  If you work on a project where
+Zuul is used to drive automation (whether that's testing proposed
+changes, building artifacts, or deploying builds), this guide will
+help you understand the concepts that underly Zuul, and how to
+configure it to meet your needs.
+
+
+.. toctree::
+   :maxdepth: 2
+
+   concepts
+   gating
+   config
+   jobs
+   encryption