Rewrite Heroku section of DD to clarify account setup and onboarding steps

[coderNini]

DD = Developer Documentation

Summary

The Heroku section is incomplete and confusing — it jumps directly into setting ENV variables without explaining account setup, leaving developers unsure of how to begin.

Proposed Changes

• Clarify whether developers need a personal Heroku account, a shared class/team account, or both
• If a shared group or class account exists for flextensions, specify how to join it and provide the necessary credentials or invite process
• If developers use a personal account, provide onboarding instructions from the very beginning (account creation, app setup, etc.) with supporting screenshots
• Add an introductory step explaining how the developer arrives at the ENV variable configuration step — what needs to happen first?

Make changes in #408 (and refer to comments for further detail)