Add docs on how to use fast-stats for specific scenarios

The fast-stats tool in my opinion has a bit of a learning curve to be able to use it effectively.

While the readme is quite comprehensive, it would be good to have some docs either in this repository or the main GitLab repository on how to use fast-stats in specific real-life scenarios.

For example, if a customer was seeing high Puma CPU usage across all of their Rails nodes, a common cause of this is a burst of web/API traffic.

• fast-stats /var/log/gitlab/gitlab-rails/api_json.log top is a good command that can show what endpoints are being hit excessively and by which users.
• What are some of the common rg/jq/grep commands we might use in conjunction with this?
• What does the fast-stats output actually mean with a real example?
• What are some of the actions we might take to resolve the problem?

There are definitely more scenarios we can come up with. This would be valuable as a reference for both Support Engineers and customers.

The inspiration is similar to this page we have already for jq https://docs.gitlab.com/ee/administration/logs/log_parsing.html

I'm not strictly decided if this should be a docs update. It could be a blog post or video.