docs: add usage of pull mirror

   gl_objects/protected_container_repositories

   gl_objects/protected_environments

   gl_objects/protected_packages

   gl_objects/pull_mirror

   gl_objects/releases

   gl_objects/runners

   gl_objects/remote_mirrors

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Project Pull Mirror

######################

Pull Mirror allow you to set up pull mirroring for a project.

References

==========

* v4 API:

  + :class:`gitlab.v4.objects.ProjectPullMirror`

  + :class:`gitlab.v4.objects.ProjectPullMirrorManager`

  + :attr:`gitlab.v4.objects.Project.pull_mirror`

* GitLab API: https://docs.gitlab.com/ce/api/pull_mirror.html

Examples

--------

Get the current pull mirror of a project::

    mirrors = project.pull_mirror.get()

Create (and enable) a remote mirror for a project::

    mirror = project.pull_mirror.create({'url': 'https://gitlab.com/example.git',

                                            'enabled': True})

Update an existing remote mirror's attributes::

    mirror.enabled = False

    mirror.only_protected_branches = True

    mirror.save()

Start an sync of the pull mirror::

  mirror.start()