docs: add instructions for syncing fork/copy with Tier IV AWSIM

[zzhhaa10000]

Checklist before requesting a review

• Follow coding standards.
• Follow AWSIM architecture.
• No unnecessary comments or Debug.Log() remain.
• The latest main branch has been merged into the work branch.
• The expected behavior is achieved after implementation. Furthermore, existing functions are not affected.
• Understand AWSIM License and agree that it applies to this contribution.

Added contents

• Explained that both fork and copy require adding upstream manually
• Added workflow for fetch/merge/rebase and pushing to origin
• Clarified default behavior of git pull/push with origin vs upstream