Add issue templates to the CE documentation

https://docs.gitlab.com/ee/customization/issue_and_merge_request_template.html exists

https://docs.gitlab.com/ce/customization/issue_and_merge_request_template.html does not

but it looks like it is open source https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/controllers/projects/templates_controller.rb

making things confusing for me https://news.ycombinator.com/item?id=12501538

1. Check if open source
2. Merge docs in CE

Thanks to @connorshea

/cc @stanhu @axil