Trigger readthedocs build for a project

**Role Variables**

.. zuul:rolevar:: rtd_project_name
   :default: ``{{ zuul.project.short_name }}``

   The readthedocs project name

.. zuul:rolevar:: rtd_webhook_id

   The readthedocs webhook API ID.  This needs to be taken from the
   project's "Integrations" dashboard page in RTD.  The URL will look
   like ``readthedocs.org/api/v2/webhook/<project-name>/<id>/``.

   This may come from a secret, however it can not be triggered
   without authentication.

.. zuul:rolevar:: rtd_credentials

   Complex argument which contains the RTD authentication credentials.
   This is expected to come from a secret.

  .. zuul:rolevar:: integration_token

     The webhook integration token.  You'll find this value on the
     project's "Integrations" dashboard page in RTD.  This can be used
     instead of username/password combo.

  .. zuul:rolevar:: username

     The readthedocs username.  If set, this will be used to
     authenticate in preference to any token set via
     ``rtd_integration_token``.

  .. zuul:rolevar:: password

     Password for ``username``.  Must be set if username is set.