Write JH frontend development guideline

We need to start to write our own JH frontend development guideline and keep improving it afterward. At the beginning, we should describe how to add JH frontend features (e.g. the render_if_exist approach), it's better to demonstrate the workflow with an example.

Upstream frontend development guideline as a reference:

• https://docs.gitlab.com/ee/development/fe_guide/
• https://gitlab.com/gitlab-org/gitlab/-/tree/master/doc/development/fe_guide