(doc) Added puppet strings generated files to gitignore
(doc) Added puppet strings generated files to gitignore

Puppet strings, the new puppetdoc reloaded project, is a wrapper on
yardoc. This commit only add doc and .yarddoc to gitignore file.

redhat-cip/puppet-openstack-cloud,gwdg/puppet-openstack-cloud,redhat-cip/puppet-openstack-cloud,gwdg/puppet-openstack-cloud,goneri/puppet-openstack-cloud,Prophidys/puppet-openstack-cloud,redhat-cip/puppet-openstack-cloud,goneri/puppet-openstack-cloud,gwdg/puppet-openstack-cloud,Prophidys/puppet-openstack-cloud,gwdg/puppet-openstack-cloud,Prophidys/puppet-openstack-cloud