Add link and/or format example of config.yml to Garbage collection topic

[bobphill]

URL: https://docs.docker.com/registry/garbage-collection.md
The format of the config.yml file, or a pointer to that format, would make the page useful

[gschaetz]

ok I got this one

[joaofnfernandes]

Reopening this since #2875 is not yet merged.