Create repo-sync.yml to enable repo-sync between repos

[CongkaiTan]

Issue #, if available:
This change aims to use the repo-sync tool to keep the two repos in sync

Description of changes:

Following the instructions listed in the repo-sync's official guide, as well as the actual implementation in the GitHub Docs repo, to create the repo-sync.yml file that defines the sync workflow:

• It runs once per day (we can change it to a higher frequency if needed)
• When new changes had been made to a repo before a run, it publishes and merges a PR

The commit history, including each commit's SHA, of the two repos will be kept exactly the same.

The secrets used by the workflow will be created in the next step.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[AWSjswinney]

Does this cherry pick commits or does it merge them? I'd really prefer that we merge from one to the other so that developers can use one clone with remotes for both destinations.

[CongkaiTan]

Does this cherry pick commits or does it merge them? I'd really prefer that we merge from one to the other so that developers can use one clone with remotes for both destinations.

Great callout. repo-sync merges the commits onto an intermediate branch, and up to this point the history is completely in sync with the source repo. However, the commits will need to be merged to main through a PR, and that's where the issues occur.

Based on a few experiments and researches, it seems through GitHub UI we can't merge the PR while keeping the history completely in sync. If we do plain merge, the original commits are left intact but we get an extra merge commit; If we do rebase, unfortunately the commits are reapplied, meaning they are essentially different commits with different checksums.

This'll also mess up the two-way auto sync, since it requires commit histories of the two repos to be exactly the same. I'll need to dig deeper and find a solution.