[doc] Add support for custom code examples

This commit adds 2 features to the documentation generation tool:
- components can use one or several 'example' blocks in the docstring
  to provide custom examples. For instance:

  """
  My nice component...
  gngngn

  .. example::
        from morse.builder import *
        toto = Toto()
        ...

  gngngng
  Blabla
  """

  Such examples are automatically moved to the 'Examples' section.

- it also add the :noautoexample: role to prevent the automatic
  generation of a code sample (which sometimes does not make sense
  at all). This role can be used anywhere in the module docstring.

These changes are demonstrated on the ArmatureSensor.