Add Puma exporter doc

What does this MR do and why?

We currently do not have a page regarding the Puma exporter as we do with other prometheus metrics. This MR adds the needed page which will need to be linked in future MRs.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.