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

doc/source/developer/index.rst

@@ -12,4 +12,5 @@ Zuul, though advanced users may find it interesting.
 
    datamodel
    drivers
+   triggers
    testing

doc/source/developer/triggers.rst

@@ -0,0 +1,19 @@
+Triggers
+========
+
+Triggers must inherit from :py:class:`~zuul.trigger.BaseTrigger` and, at a minimum,
+implement the :py:meth:`~zuul.trigger.BaseTrigger.getEventFilters` method.
+
+.. autoclass:: zuul.trigger.BaseTrigger
+   :members:
+
+Current list of triggers are:
+
+.. autoclass:: zuul.driver.gerrit.gerrittrigger.GerritTrigger
+   :members:
+
+.. autoclass:: zuul.driver.timer.timertrigger.TimerTrigger
+   :members:
+
+.. autoclass:: zuul.driver.zuul.zuultrigger.ZuulTrigger
+   :members:

doc/source/index.rst

@@ -24,7 +24,7 @@ Contents:
    executors
    statsd
    client
-   developer
+   developer/index
 
 Indices and tables
 ==================