Generic docs need reST

Conversion of all docs to reST so that the generic docs can link easily
with the rest of the docs.

docs/api_generic.rst

@@ -0,0 +1,87 @@
+===============
+Generic Devices
+===============
+
+.. currentmodule:: gpiozero
+
+The GPIO Zero class hierarchy is quite extensive. It contains a couple of base
+classes:
+
+* :class:`GPIODevice` for individual devices that attach to a single GPIO pin
+
+* :class:`CompositeDevice` for devices composed of multiple other devices like
+  HATs
+
+There are also a couple of `mixin classes`_:
+
+* :class:`ValuesMixin` which defines the ``values`` properties; there is rarely
+  a need to use this as the base classes mentioned above both include it
+  (so all classes in GPIO Zero include the ``values`` property)
+
+* :class:`SourceMixin` which defines the ``source`` property; this is generally
+  included in novel output device classes
+
+.. _mixin classes: https://en.wikipedia.org/wiki/Mixin
+
+The current class hierarchies are displayed below. For brevity, the mixin
+classes are omitted:
+
+.. image:: images/gpio_device_hierarchy.*
+
+.. image:: images/composite_device_hierarchy.*
+
+Finally, for composite devices, the following chart shows which devices are
+composed of which other devices:
+
+.. image:: images/composed_devices.*
+
+Base Classes
+============
+
+.. autoclass:: GPIODevice(pin)
+    :inherited-members:
+    :members:
+
+.. autoclass:: CompositeDevice
+    :inherited-members:
+    :members:
+
+Input Devices
+=============
+
+.. autoclass:: InputDevice(pin, pull_up=False)
+    :members:
+
+.. autoclass:: WaitableInputDevice
+    :members:
+
+.. autoclass:: DigitalInputDevice(pin, pull_up=False, bounce_time=None)
+    :members:
+
+.. autoclass:: SmoothedInputDevice
+    :members:
+
+.. autoclass:: AnalogInputDevice
+    :members:
+
+Output Devices
+==============
+
+.. autoclass:: OutputDevice(pin, active_high=True)
+    :members:
+
+.. autoclass:: PWMOutputDevice(pin, frequency=100)
+    :members:
+
+.. autoclass:: DigitalOutputDevice(pin, active_high=True)
+    :members:
+
+Mixin Classes
+=============
+
+.. autoclass:: ValuesMixin(...)
+    :members:
+
+.. autoclass:: SourceMixin(...)
+    :members:
+