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/admin/connections.rst b/doc/source/admin/connections.rst
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+:title: Connection Configuration
+
+.. _connection-config:
+
+Connection Configuration
+========================
+
+Most of Zuul's configuration is contained in the git repositories upon
+which Zuul operates, however, some configuration outside of git
+repositories is still required to bootstrap the system.  This includes
+information on connections between Zuul and other systems, as well as
+identifying the projects Zuul uses.
+
+.. _connections:
+
+Connections
+-----------
+
+In order to interact with external systems, Zuul must have a
+*connection* to that system configured.  Zuul includes a number of
+drivers, each of which implements the functionality necessary to
+connect to a system.  Each connection in Zuul is associated with a
+driver.
+
+To configure a connection in Zuul, select a unique name for the
+connection and add a section to **zuul.conf** with the form
+"[connection NAME]".  For example, a connection to a gerrit server may
+appear as::
+
+  [connection mygerritserver]
+  driver=gerrit
+  server=review.example.com
+
+.. _drivers:
+
+Drivers
+-------
+
+Drivers may support any of the following functions:
+
+* Sources -- hosts git repositories for projects.  Zuul can clone git
+  repos for projects and fetch refs.
+* Triggers -- emits events to which Zuul may respond.  Triggers are
+  configured in pipelines to cause changes or other refs to be
+  enqueued.
+* Reporters -- outputs information when a pipeline is finished
+  processing an item.
+
+Zuul includes the following drivers:
+
+.. toctree::
+   :maxdepth: 2
+
+   drivers/gerrit
+   drivers/github
+   drivers/smtp
+   drivers/sql
+   drivers/timer
+   drivers/zuul