Add group-level project templates dev guideline docs

What does this MR do and why?

Add group-level project templates dev guideline docs

This is the first iteration which documents the basic flow and all of the related classes.

In a later iteration we should add flow diagrams explaining how the code is executed.

Closes #424528 (closed)

Screenshots or screen recordings

How to set up and validate locally

Load gdk and visit /help/development/project_templates/index