Update Docs for explaining remote_development best practice

Issue: Update Docs for explaining remote_development b... (#476635 - closed)

What does this MR do and why?

This MR is only doc changes for workspace rails. It includes below 4 parts:

• explain different levels of testing in doc
• explain Ruby superclass and mixin and class method usage with remote_development
• fix mermaid chart issue (not rendering before)
• removed one small incomplete text