[Step 1] Add documentation about how to reduce snippet repository size
Since snippets repository size will be part of the namespace's quota, it's important to inform users how to try to reduce that size. Even more when we don't have (yet) any housekeeping mechanism over those repositories.
We need to add some documentation about this. Maybe https://docs.gitlab.com/ee/user/snippets.html#versioned-snippets should be the place to include it. Since snippet repositories don't allow to create other branches nor tags, there are only a few things we can do for this task. The necessary info is in https://docs.gitlab.com/ee/user/project/repository/reducing_the_repo_size_using_git.html#purge-files-from-repository-history.
I'm not sure whether we should link to that section or rewrite the information there and show only the possible things users can perform on snippet repositories.
/cc @phikai