Add tutorial on forking Geode profiles

[jeffs]

Users sharing a Geode profile inevitably want different settings: The user name and profile in docker.toml, user-specific tool config (e.g., the email address in gitconfig files), etc. The result of this ticket should be a tutorial to help users:

• Fork an existing profile
  • Make and version user-specific changes
• Contribute changes upstream (e.g., via Pull Request)
• Fetch upstream changes
  • Re-base local changes onto the main branch
  • Rebuild image, and possibly reinitialize home volume