Evaluate and address concerns for Puma thread tunings

[aneta-petrova]

          I still have a strong opinion that users shouldn't touch these values if we can avoid it. The values are automatically determined and if that's incorrect then we should fix it. For example, https://github.com/theforeman/puppet-foreman/pull/1197.

My preference would be to drop this whole chapter or make it more like Puma's official documentation (https://msp-greg.github.io/puma/#thread-pool) with specific recommendations for our application.

Originally posted by @ekohl in #3248 (review)

[aneta-petrova]

#3248 was merged after being acked but it looks like there are some concerns regarding the currently-documented approach. I'm creating this issue to track and, if needed, address them.

[ekohl]

In #3534 I made a start