Update "Keeping your fork in sync" instructions in dev workflow doc to favor rebase

[lukpueh]

The "Submit a pull request." section of our dev workflow doc points to GitHub docs that explain how to keep your fork in sync. It looks like GitHub updated their instruction to use merge instead of rebase, which conflicts with our recommendation in our git history guidelines, which strongly favor rebase.

Possible Solutions:

1. Remove link to GitHub doc
2. Replace it with a link to instructions that favor rebase
3. Add a note to follow the instructions only until before the merge and to then use rebase instead of merge