Make the "this is technical documentation" message of "Arcanist Overview" more clear

Summary:
The only purpose of "Arcanist Overview" is to tell people they shouldn't be here, but we bury the lede.

Make it clear that this is not user documentation.

After T988 we can improve the organization here, but some recent users found this pretty confusing.

Test Plan: Generated and read documentation.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3235

.divinerconfig

@@ -3,7 +3,7 @@
   "src_link" :
     "https://secure.phabricator.com/diffusion/ARC/browse/master/%f$%l",
   "groups" : {
-    "intro" : "Introduction",
+    "notuser" : "HEY! THIS IS NOT USER DOCUMENTATION!",
     "config" : "Setup & Configuration",
     "workflow" : "Workflows",
     "lint" : "Lint Integration",

src/docs/notforusers.diviner

@@ -0,0 +1,8 @@
+@title This Is Technical Documentation For Contributors, Not User Documentation
+@group notuser
+
+This section of the documentation has technical information for developers
+and contributors.
+
+For user documentation, see the
+[[http://www.phabricator.com/docs/phabricator/ | Phabricator Documentation]].

src/docs/overview.diviner

@@ -1,24 +0,0 @@
-@title Arcanist Overview
-@group intro
-
-Overview of Arcanist, a code workflow tool.
-
-Arcanist (commonly, "arc") is the command-line frontend to Phabricator and
-Differential. A detailed command reference is available by running ##arc help##.
-
-= Documentation =
-
-The Arcanist documentation is primarily focused at Arcanist developers and
-explains the project's internals. Arcanist user documentation which explains
-how to use the tool is available in the Phabricator project:
-
-  - for an overview of Arcanist, see @{article@phabricator:Arcanist User
-    Guide}.
-  - for information on configuring a new project and setting up an
-    ##.arcconfig## file, see @{article@phabricator:Arcanist User Guide:
-    Configuring a New Project}.
-  - to install remote hooks in a repository, see @{article@phabricator:Arcanist
-    User Guide: Repository Hooks}.
-  - to integrate Arcanist with linters, unit tests and custom workflows, see
-    @{article@phabricator:Arcanist User Guide: Customizing Lint, Unit Tests and
-    Workflows}.