[Doc] How to add a new Gem to JH

Related issue: #2932 (closed)

What does this MR do and why?

Add a new doc: How to add a new Gem to JH.

Screenshots or screen recordings

Non UI

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.