Expanded onboarding draft with more detail and examples

* Added Git info and GitHub account verification
* Explained good first issues
* Explained origin and upstream differences
* Explained syncing forks
* Encouraged running tests before and after changes
* Gave example branch names and commit messages
* Recommended starting with Draft PR, with screenshots
* Detailed CI process
* Encouraged community engagement
* Added TODOs:
  * Brief build instructions or troubleshooting
  * Review process timeline and how to proceed if no feedback is
    received
  * FAQ

Author: jhelsby