Document what a 'Sidekiq Shard' is (GitLab.com terminology)

The following discussion from !35048 should be addressed:

• @cmiskell started a discussion: (+1 comment)

  I like this simple phrasing of what a shard is. I shall likely borrow it. But I too feel like we need to define this, perhaps somewhere else because it's gitlab.com specific terminology. I've added some bits to gitlab-com/runbooks!2383 (merged) and while we could maybe link to parts of that when done, it's not for the same audience. Not sure where else it could go though.