Rewrite Rebasing Guidelines with Contributors as Target Audience

[JakeQZ]

The current section in CONTRIBUTING.md on rebasing was written based on the experience of rebasing as a collaborator.

Colloborators can create branches within the repository for PRs, and don't need to fork it. Contributors, however, will need to make a forked copy for their PR. Thus, the examples and other information given in this section may not be consistent with their workflow.

The target audience for this section should be contributors (not collaborators), and it should be updated to reflect that.