Add description of how to import a bare repo after Rake task removal in 16
We removed a Rake task in 16 (https://docs.gitlab.com/ee/raketasks/import.html) but didn't provide (or link to? maybe I just missed it) an alternative approach. It's fairly straight forward, but if we don't already have it somewhere in the docs, we should add the steps and link to it from the deprecated Rake task page.