Create content-translation-template

[kaitohm]

I'm creating a new issue type to be used for those wanting to translate content that already exists on Learn WordPress.

An accompanying Handbook draft can be found here: https://docs.google.com/document/d/1wuPlPVjVypxCzajrsnLWvQKqsyM3jf-x3WWcv2BFgvw/edit?usp=sharing