Add zuul from scratch doc

This is just a skeleton for now, and should be expanded, but it
should be correct and up to date for what we expect in the v3
release.

Change-Id: I305200cbfc128cc1bc09e809540a873f60e0fe5c
Co-Authored-By: Leif Madsen <lmadsen@redhat.com>
Co-Authored-By: Monty Taylor <mordred@inaugust.com>
Co-Authored-By: Paul Belanger <pabelanger@redhat.com>
diff --git a/doc/source/admin/index.rst b/doc/source/admin/index.rst
index a2a2ee7..af83a3b 100644
--- a/doc/source/admin/index.rst
+++ b/doc/source/admin/index.rst
@@ -12,6 +12,7 @@
    :maxdepth: 2
 
    quick-start
+   zuul-from-scratch
    installation
    components
    connections