Add trigger developer doc

Move developer docs to their own subdir (to avoid filename conflicts
with current doc set), and add a (very basic) new dev doc for triggers.

Change-Id: I0dce63ab9fa470247c4bbd4aae58dc97cdf8a517
diff --git a/doc/source/developer/index.rst b/doc/source/developer/index.rst
new file mode 100644
index 0000000..986bbe4
--- /dev/null
+++ b/doc/source/developer/index.rst
@@ -0,0 +1,16 @@
+Developer's Guide
+=================
+
+This section contains information for Developers who wish to work on
+Zuul itself.  This information is not necessary for the operation of
+Zuul, though advanced users may find it interesting.
+
+.. autoclass:: zuul.scheduler.Scheduler
+
+.. toctree::
+   :maxdepth: 1
+
+   datamodel
+   drivers
+   triggers
+   testing